XCOM: Enemy Unknown Impossible Difficulty Guide (Our Finest Hour)

This guide will give you some useful tips on beating XCOM: Enemy Unknown on Impossible difficulty without Ironman mode and without using cheats, glitches or exploits. This can also be used as a general guide for playing XCOM Enemy Unknown although you have to play it once to understand the basics. This is not a guide for the Enemy Within expansion or Ironman mode as well as other difficulty parameters.

[1.0] Introduction
[2.0] Difficulty differences
[3.0] Save/Load Game System
[4.0] Basics
[5.0] Research and Engineering
[6.0] Barracks
........[6.1] Squad Selection
........[6.2] Assault
........[6.3] Heavy
........[6.4] Support
........[6.5] Sniper
........[6.6] Psi soldier
........[6.7] Hiring
........[6.8] Officer Training School
[7.0] Situation Room and Mission Control
[8.0] Ground Tactics
........[8.1] Scouting
........[8.2] Will management
........[8.3] Flanking
........[8.4] Other tips
[9.0] Conclusion

[1.0] Introduction:
An ending few people have seen
XCOM: Enemy Unknown is a turn-based tactical strategy game. A must play for those who love tactical games. The story is based on a sci-fi world involving aliens and has some great music made by Michael McCann who is also the composer of other great titles like Deus Ex: Human Revolution and Splinter Cell: Double Agent. It is also one of those games where the difficulty lies inside the game and not on pressing the buttons with twitch timing. You cannot keep grinding too like in some RPGs because the enemy does not wait. Losing in this game depends on everything you do from the start of the game and not just the most recent mission which makes it a lot different than other games where you can just reload a previous save game and continue. It has a reverse difficulty curve. Meaning the start of the game is incredibly difficult and it becomes easier late in the game. Except that it will become easy only if you manage to not keep losing countries at the start of the game. 16 countries fund the XCOM project. They will withdraw funding the project if you fail to help those countries which it turn increases the difficulty. It will be Game Over if you lose 8 or more countries. One of the best things about this game is that most of the stuff other than the main story events is random – the maps are random, the enemies are random, the enemy spawn points are random. Because of this a complete walkthrough cannot be made for this game.
To begin with let us see some stats first:
As of June 2015, 27.8% have finished the game whereas only 1.1% has finished the game on impossible difficulty. That means only 3.96% people who were interested in finishing the game beat it on impossible difficulty. Even if you consider that most people won’t want to play the game twice, 3.96% is a lot less which indicates that people are finding it really hard to beat it at impossible difficulty. Surprisingly [Classic + Ironman] is easier than Impossible without Ironman mode which allows you to abuse the save system or maybe more people were just interested in beating the game at Classic difficulty. Beating the impossible difficulty is actually quite easy really. Beating it without losing any country, soldier and mission is hard and also requires a bit of luck. Beating it on Ironman mode though is another story.

[2.0] Difficulty differences
The following are the major differences in each difficulty level (Easy/Normal/Classic/Impossible):
1. Initial global panic per country: 0/0/1/2
2. Initial funding(§): 175/175/100/75
3. Initial Power: 42/37/30/30
4. Soldier cost(§): 10/10/15/15
5. Soldier starting HP: 6/5/4/3
6. On easy and normal there is a satellite in reserve and Officer Training School is already built.
7. On easy monthly income is increased by 50%.
8. On classic and impossible fewer engineers and scientists are given per satellite launch.
9. Alien activity causes more panic in countries with each higher difficulty level.
10. Enemies have higher stats (HP, Hit, Critical chance, Will, etc) with each higher difficulty level.
11. More enemies on map with each higher difficulty level.

That should give you an idea of how impossible difficulty looks like. If your math is bad then just look at the simple image below that shows the resources recovered after a mission.
The map had 5 Sectopods. Doesn’t tell much right? How about 5 Sectopods visible at the same time with all attacking your squad in the same turn? Yeah that’s impossible difficulty. Classic difficulty is a walk in the park by comparison.

[3.0] Save/Load Game System
This isn’t Ironman mode. So you can use the save/load system all you want. Abuse it. But there is a bug in the load game menu. If you exceed 99 save files the sorting order gets messed up in the menu. The developers probably limited the save files on the consoles due to limited storage space. You can get over this issue by two ways:

Delete older save files manually inside load game menu. This is a little tiresome but this lets you see details of the save file while deleting it so you don’t end up deleting an important save file. If you do not care about older save files (or have backed them up) then you can just simply keep on deleting the first entry till the menu gets sorted out by having less than 99 files.

Or navigate to “C:\Users\[Username]\Documents\My Games\XCOM – Enemy Unknown\XComGame\SaveData”
See the save files there. Don’t go by the file names. They get messed up once you start deleting files. Sort them by Date Modified. Keep the latest files and delete or back up the remaining files. If you delete 90 of them you will have no problems with the load game menu for the next 90 saves. Keep doing this whenever you hit 99 saves.

One thing to keep in mind when using the save/load system is that the game calculates the hit and critical chances for everyone at start of each turn. So you need to save at the end of previous turn to alter the outcome. Just keep saving every successful turn. If you forgot to manually save an important turn the game auto-saves the last 3 turns. Using the save/load system is the most OP thing to do but if you end up reloading saves 10 times for one turn then you are wasting time. The rest of the guide will tell you how to play efficiently at the hardest difficulty without wasting lots of time deciding what to do next.

[4.0] Basics
You won’t notice the huge brain size of the Sectoid until you play at the Impossible difficulty
To understand basics of the game just play the game on easy or normal difficulty with the tutorial on. If you can’t beat the game at easy on your own then don’t even think about beating the game on impossible. You will just get terribly frustrated when you see a Sectoid kill your Heavy with full HP in full cover in one shot. Play the game on easy. Play it for fun. Understand the game mechanics. See what all stuff unlocks. And see what the endgame looks like. The game has a reverse difficulty curve. The game will become easier as you progress if and only if you play it right otherwise it will be just as difficult as the start. This guide isn’t written to tell you the basics of the game but to tell you how to play it right and what matters at the impossible difficulty. One thing to understand is that at the impossible difficulty you are going to lose soldiers and lose countries. Don’t cry over it. The thing that you need to do is to minimize losses as much as you can. Do not lose more than one soldier at the start of a map. If you happen to lose 2 or more you are likely to lose the entire squad. Reload a previous save and play better. If you are playing with Ironman on then retreat back to the Skyranger. If you are feel you are being overwhelmed just reload the save before launching the map and just ignore the mission. You might end up losing a country at the end of the month but that’s all fine as long as it’s just one country. If you happen to lose more than one country per month then you need to play better.

[5.0] Research and Engineering
Dr. Vahlen sure loves her XPad 24x7
Give the lowest priority to the objectives marked as *PRIORITY*. The priority objectives progress the story and you will face tougher enemies when you do that. So complete all other objectives that you wish to complete before doing the priority one. Another important thing is multitask. Multitask efficiently. For example, a satellite takes 20 days to manufacture and satellite uplink takes 14(7 with Advanced Construction) days to manufacture. One would think let’s make a satellite uplink first since there is no point in making satellite when there’s no uplink. Doing so will make you wait 20 additional days for getting a satellite in orbit. That’s a big loss. You need to start manufacturing so that they end up getting manufactured the same day so that you can deploy a satellite right away. Also if you manage to deploy just before the end of a month it would be a huge bonus since you will get credit from the country where it was deployed. Timing is everything. And multitask like this for everything.

Talking about satellites, satellites are the most important thing to make. Give it the highest priority than everything else. It does two things: it gives you funding, country specific bonuses and it also decreases the panic level of the country. There are 4 things to consider: Satellite, Satellite Uplink/Nexus, Stealth Satellites, and Firestorm. The most effective way to cover all 16 countries is to have 2 Uplinks and 2 Nexuses in a square. But you are probably going to lose some countries so you might not have to build 2 Nexuses.
XCOM’s satellites work underground!
3 Uplinks = 8 Satellites
2 Uplinks + 1 Nexus = 10 Satellites
4 Uplinks = 12 Satellites
3 Uplinks + 1 Nexus = 14 Satellites
2 Uplinks + 2 Nexuses = 16 Satellites

You will have to figure out how many countries you can save and build Uplinks/Nexuses accordingly. Since a satellite decreases the panic in the country where it is deployed you might want to check the panic level of all countries before considering a specific country’s bonus when a satellite is deployed. Now the problem is after all this hard work an alien UFO can take out your satellite. This is really problematic since this is a double loss. To counter this you also need to put priority on Stealth Satellites and Firestorm. You can take out most of the UFOs with 2 interceptors (+ Dodge, Boost, Aim) but it will be pathetic against large ones and you won’t have interceptors ready every time a UFO is spotted. Get 2 Raven interceptors on every continent as fast as you can since the Firestorm comes mid-game.

There will be a need of more engineers as you start building facilities so give priority to building Workshops too. Building laboratories can be skipped entirely. You can get the required scientists from a mission reward if you do need them badly. But don’t waste a facility slot for a lab.

Don’t skip laser weapons. One might think that skipping laser weapons would allow you to save resources and time for other stuff but laser weapons are important on the battlefield. Those 2 pieces of extra damage are going to be a lot more helpful on the battlefield on the impossible difficulty.

S.H.I.V. can be skipped. S.H.I.V. comes somewhere mid-game and just a little later you will get Psi soldiers. Psi soldiers are a lot better than S.H.I.V. so save the resources for something else rather than building S.H.I.V.s. Also a Psi soldier is necessary to complete the game whereas S.H.I.V. is not.


[6.0] Barracks

[6.1] Squad Selection
Assault, Heavy, Support, Sniper – you need each one of them for every mission. If you have the squad size upgrades then the other two soldiers can be anything. I say anything because it would depend on what soldiers you get and which soldiers are ready for the mission. You can have e.g. 1 assault, 1 sniper, 1 support and 3 Heavies. That combination is really good against maps with Sectopods and Cyberdiscs. Also one more important thing to keep in mind while choosing a squad: choose at least one lowest ranked soldier for your squad for every mission.

In the following sections preferred abilities are suggested. You get to choose between 2 different abilities as your soldier ranks up. In general both abilities are equally good. But certain combinations of abilities work well together which are suggested below. The key tactic here is to choose abilities that work more predictably. There are too many random factors in XCOM’s gameplay, the better you can predict the outcome the more easily you can play the game. Armor choices aren’t mentioned since they will be based on what you have researched. Ghost, Psi Armor comes very late in the game when you already have a good chance at winning.

[6.2] Assault
Tactical Sense vs. Aggression: Tactical Sense should be the preferred choice. You need more defence that crit when surrounded by enemies. Otherwise the next turn your assault is dead. And don’t count on the rest of the soldiers finishing the other enemies at the impossible difficulty.

Lightning Reflexes vs. Close & Personal: Lightning Reflexes works well with the default Run & Gun ability. Let the assault become bait to any Overwatch by the enemies. And it gives a better chance to close in on the enemy with a shotgun. Assault already has good damage potential at close range so Close & Personal has a lesser advantage than Lightning Reflexes.

Flush vs. Rapid Fire: When you are close to an enemy with a shotgun type weapon, I mean really close like on the adjacent square to a Muton then you will have a 100% hit chance even on impossible difficulty. Rapid Fire carries a -15 penalty to Aim but up close the penalty won’t be huge giving you 2 hits with the shotgun on the enemy. Flush on the other hand does very little damage and requires your other soldiers to be on Overwatch when they can do something else more productive. Moreover the enemy might just get behind another cover.

Close Combat Specialist vs. Bring ‘Em On: Close Combat Specialist will save you on a lot of turns especially with melee enemies like Chryssalids and Berserkers. Also if you can make the enemy come to you by hiding behind a corner it’s a huge advantage. And since the ability is triggered when the enemy comes within 4 tiles it works well with a shotgun type weapon with a huge Hit & Crit Chance. Bring ‘Em On adds damage on critical hits but the chances of critical hit is low already not to forget the low chances of an actual hit.

Resilience vs. Killer Instinct: Again you need more defence that crit. So resilience is a good choice. Because at impossible difficulty enemies crit a lot and your Assault’s hit chances are low.

Equipment: For early game Nano-Fiber vest, chitin plating or grenades are good. After you get the Titan Armor which gives a bonus with extra conditioning you can keep carrying grenades. Grenades are a lot more useful at the impossible difficulty since you know for sure whether it will hit or not and also breaks the enemy’s cover in most situations.

[6.3] Heavy
Bullet Swarm vs. Holo-Targeting: With bullet swarm firing the primary weapon doesn’t end the turn. You can fire again, move somewhere, go Overwatch or reload or even throw a grenade. That’s a lot of advantage over Holo-Targeting.

Shredder Rocket vs. Suppression: The enemies have a lot more HP at impossible difficulty and most of the time you won’t be able to take them down in one turn. Shredder Rocket will help to deal more damage to the enemies. And if you have 2 or more Heavies in your squad one can fire a Shredder rocket and the other can fire a normal rocket to finish them off. Especially useful if you find them in groups without noticing you by throwing a Sniper’s Battlescanner.

HEAT Ammo vs. Rapid Reaction: Rapid Reaction grants you a second reaction shot on over watch if the first reaction shot was successful. Now what are the chances of the first reaction shot to be hit? The chances of a normal shot itself is too less at impossible difficulty. That is enough reason to avoid choosing Rapid Reaction. HEAT Ammo is good against robotic enemies and you do need it since they have very high HP.

Grenadier vs. Danger Zone: Although using explosives deny you of salvage, grenades are a life saver. You will have to do one or two missions more to get required salvage for engineering in some cases but most of the time you will be waiting on something else anyways. And if you use grenades first to break cover and damage a little and shoot them after, getting salvage won’t be a problem. There are 3 important benefits of using grenades: it has a 100% hit chance, it can hit more than one enemy, and it can break cover. You can’t ignore these benefits. Although you will get only 2 grenades with each Heavy it is enough for critical situations where most of your soldiers aren’t able to hit the enemies successfully. Danger zone adds 2 tile radius to your rockets but that isn’t much of a benefit.

Rocketeer vs. Mayhem: The numbers of enemy spawn points are much higher at impossible difficulty. You need more rockets than more damage in a single shot. Rocketeer is the preferred choice.

Equipment: Grenades. Works well with Grenadier ability. Enough said.

[6.4] Support
Sprinter vs. Covering Fire: Moving 3 additional tiles is really useful in some situations. Especially useful when an injured soldier is a little far away and need to heal him/her. Or maybe you need to flank an enemy. You shouldn’t choose covering fire since normal shots themselves miss a lot.

Field Medic vs. Smoke and Mirrors: Some maps are long and you will face lots of enemies on impossible difficulty. Getting to use Medikit 3 times is a godsend. On the other hand using smoke and mirrors gives you 1 additional use of smoke which by itself isn’t of much use. Remember that some aliens have grenades and grenades have 100% hit chance. Using smoke and gathering your soldiers in that radius is going to make a grenade attack inevitable.

Revive vs. Rifle Suppression: You really don’t want to lose a soldier at impossible difficulty. And with revive you can get the soldier back up in the fight which is awesome. Rifle Suppression again isn’t going to be of much use as the enemies somehow do critical hits even with aim penalty at impossible difficulty.

Dense Smoke vs. Combat Drugs: This is a hard choice. Combat drugs are useful against Psionic attacks but they come late in the game. But you probably own the aliens by then. If the Ethereal gets to mind control one of your soldiers you might be in a real bad situation. Combat Drugs can be useful in such situations. Dense Smoke on the other hand finds its use almost everywhere.

Savior vs. Sentinel: With improved Medikit Savior will heal up to 10 units which is just awesome since enemies do critical hits a lot. Sentinel’s use depends on whether the reaction shot lands or not. The chances are pretty low on impossible difficulty so it’s better to choose something that is guaranteed to work.

Equipment: Medikit no doubt. Support is the perfect candidate to use an arc thrower. Also with the increased movement with Sprinter the support can close in on the enemy in one turn and use the arc thrower. After arc thrower becomes unnecessary late game, you can choose between mind shield or Combat Stims or grenade. It depends on your play style.

[6.5] Sniper
Snap Shot vs. Squadsight: This is one of the things that is going to differ from other guides. According to me both have their advantages and disadvantages. Having 2 snipers in your team with both abilities is a better idea. Most other guides will tell you that Squadsight is completely OP. What they won’t tell you is that for Squadsight to work it requires unobstructed line-of-sight (LoS) to the target. It’s really difficult to have LoS from a far distance (which is the point of using Squadsight in the first place) and you will be wasting the sniper’s turns to get LoS. A Squadsight Sniper usually ends up useless most of the time. Mind you multiple enemy groups will start actively looking for you at the impossible difficulty and you won’t have time to waste turns positioning your sniper. Also positioning the Sniper for LoS requires you to know the target’s position in advance which is really impossible unless you have Psionic powers for real. Also most of the other abilities like Battlescanner, In the Zone, etc won’t work well with a Squadsight Sniper. Add to the fact that Squadsight replaces Snap Shot that can be used to position the sniper. You will be wasting a lot of time with a Squadsight sniper and this guide is about beating the game efficiently in the least time as possible. One more drawback of Squadsight ability is that if the soldier happens to become a Psi soldier, Psi abilities don’t work with Squadsight. Even though Snap Shot gives an Aim penalty, you can’t actually expect to shoot enemies from the front at impossible difficulty. You have to flank as much as possible and Snap Shot helps with it. Flanking and shooting will give you more hit chance than without the aim penalty.

Gunslinger vs. Damn Good Ground: Sometimes a sniper can get in close proximity to an enemy like floaters for example that can move to anywhere on the battlefield. Having more permanent pistol damage is good than the temporary bonus of damn good ground while at elevation. Also when you run out of Sniper Ammo in the heat of battle (which will happen more often than you think) you can switch to a good Pistol.

Disabling Shot vs. Battle Scanner: Battle Scanner is useful if you throw it in the right spot. If the enemies don’t notice you, your Heavy can take them down with rockets. Battle scanner can also be used to safely flank an enemy so that you can avoid alerting another enemy group. Disabling Shot just wastes one turn of the enemy and so are you.

Executioner vs. Opportunist: Executioner confers +10 Aim against targets with less than 50% health. The sniper’s damage is one of the highest and you won’t want to waste such high damage on an enemy with low health. Opportunist gives a high hit chance with reaction shots and is highly recommended.

Double Tap vs. In The Zone: In the Zone is the most OP ability in the game. It requires flanking (or exposing) and killing the enemy. You can finish off the entire enemy group if played properly. For example, in maps where there are only Thin Men, the last round of Thin Men drops on the ground fully exposed and goes to Overwatch. All of them can be killed with just one properly positioned Sniper in one turn. Also a lot of other enemies like Chryssalids, Zombie, Berserkers and especially Drones don’t use cover and they can be killed without losing the Sniper’s turn. With Snap Shot you can get to hit multiple enemies from two positions. Other soldiers can help by exposing the enemy using explosives and initially damaging high HP targets.

Equipment: Scope. You don’t want to miss your sniper’s shots.

[6.6] Psi soldier
Psi Inspiration vs. Psi Panic: Psi Panic is quite unpredictable. The enemy might end up hitting one of your soldiers. There is absolutely no doubt here – Psi Inspiration is recommended. Psi Inspiration will allow you to increase your soldiers Will before attempting difficult Psi attacks.

Telekinetic Field vs. Mind Control: Although Telekinetic Field is a powerful ability it doesn’t offer a tactical advantage as good as Mind Control. Psi attacks have a huge range and if your soldier has enough Will it will have a high Hit chance. You can Mind Control someone far behind in an enemy group so as to gain a lot of vision. You can use that soldier now to kill the enemies in his front completely flanked. You can make him as bait so that your soldiers won’t get hurt. And if there are turns remaining you can use him to easily scout ahead.

Equipment: If your soldier lacks Will, Mind Shield is a must. Otherwise the suggested equipment for the specific class is good enough.

[6.7] Hiring
There was never a real requirement to hire soldiers at the easy difficulty level. But at impossible soldiers die and you will be running out of soldiers sooner than you think. Whenever you have the credits hire soldiers (15§). 18 soldiers should be enough to keep back to back missions going. You can stop hiring when you have 18 soldiers alive. You might want to hire more for farming Psi soldiers late in the game. Having at least 3 Psi soldiers with good enough Will is recommended.
Remember Titan Armor costs 150§ whereas a soldier costs 15§. So hire away!

[6.8] Officer Training School
Squad Size I and II are the most important Officer Training School upgrades.

[7.0] Situation Room and Mission Control
As already said Satellites should be deployed in countries with the highest panic level before considering the country or continent specific bonuses.
As for choosing mission rewards the priority should be as follows:
Panic Level > Engineers > Soldier > Credits > Difficulty > Scientists

The soldier granted as a reward for missions is really important since it will be a ranked soldier with likely having higher Will. When you have enough Engineers at the end of the game you can give Soldiers as a reward more priority.

When you press the Start Scan button a Stop Scanning button will replace it. Use it. Some events don’t alert the player while scanning.

[8.0] Ground Tactics

[8.1] Scouting
The first thing to do when you land on a map is scout. The rules are simple. Don’t split, don’t spread, and don’t stick. Splitting up is a bad idea. You can’t take on the enemies with half of your squad. Don’t split unless you know for sure the enemy is completely alone and cornered like the Outsider. Spreading out is also a bad idea. One of your soldiers at the far end might likely alert an enemy group and your other soldiers might be too far away to help him/her in time. You should spread out in a way that each of your soldiers is able to reach every other soldier in your squad. Especially the support soldier so that he/she can heal/revive in the next turn. Sticking to each other will make a grenade or some AoE attack on your squad inevitable. In an open map always scout around the edges of the map. Don’t go in the centre, you will likely get ambushed by aliens from multiple sides. Also don’t dash when scouting, always use one turn to move to a cover ahead for the entire squad first and then go to Overwatch even if there are no enemies nearby.

[8.2] Will management
Soldiers panic. And that is a huge problem. Not just that you will lose a soldier’s turn but he/she might hit somebody else in your squad who can even die. To reduce panic you will need to manage Will. One of the stats that look completely irrelevant at the easy game difficulty just becomes the most important stat at the impossible difficulty. Late game when Psionics comes into play Will becomes the most important stat. Your soldiers will easily fall victim to Psi attacks and your Psi soldiers won’t be able to use Psi attacks successfully. Things to keep in mind to manage Will:

  • ■ When a soldier gains a rank, there is a randomized 2-6 point Will increase.
    ■ Critically wounded soldiers, even if saved, incur a permanent -10 (-15 on Classic or Impossible) reduction to Will.
    ■ Fellow squad mates are subject to a "Fallen Comrade" penalty of -5 Will if they witness a soldier being killed in action during a mission. In the event of multiple deaths, the penalty is -5 Will per soldier killed.
    ■ If a soldier is injured during combat, for the remainder of the mission they are subject to a Will penalty, visible as "Battle Fatigue" when viewing the soldier info interface. If injured by less than 50% of their total health, the penalty is -5 Will. If injured by more than 50% of their total health, the penalty is -10 Will.
    ■ Note: Whenever a soldier incurs damage exceeding that of the HP bonus granted by their armor, the soldier is flagged as wounded.
Basically you have to keep soldiers from getting injured as much as you can. You can lose the armor bonus. Use a Medikit cleverly. Also keep deaths to a minimum (like a maximum of 7 for the whole campaign). Psi attacks are the best and most powerful attacks in the game. Psi attacks ignore cover, have pretty good range, and are always almost a 100% hit if your soldier has a Will above 90 and offers a huge tactical advantage. Only Ethereal and Berserkers will be able to resist Psi attacks from a soldier with high Will. But Ethereal and Berserker usually comes in groups with Mutons or Muton Elites which can be Mind Controlled. Robotic enemies are of course immune.

[8.3] Flanking
The chances of taking a shot at an enemy in full cover or even partial cover is more likely going to be a miss at early stages at impossible difficulty. Flanking is necessary not an option. But there is another problem. At impossible difficulty there are much more enemies on the map. There is a high chance that when you move one of your soldiers to go flank an enemy you might alert another enemy group. This is worse than not flanking at all! In such cases where there is a possibility of alerting another enemy group its best to break the enemy’s cover with explosives. You can also use the Assault’s Run & Gun ability to just move up to the enemy’s adjacent square and hit him if the rest of the enemies can be eliminated by others. It’s a little risky but if done right you can get away without alerting another enemy group. If you are playing on Ironman mode you might want to throw a Battlescanner to see if the flanking area is clear.

[8.4] Other tips

  • ■ Don’t forget to reload.
    ■ Move the Assault with Lightning Reflexes first during your turn so that he/she becomes bait to any of the enemies’ Overwatch.
    ■ Whenever possible give the killing shot to your lowest ranked soldier in your squad. Actually when the risk is low you should try to give the kill shot to that soldier even if it takes a few turns to do so. This will help you to get more ranked soldiers in your barracks for later use. Always remember your best soldier can get wounded for a long time or end up dead and you will have to use the other soldiers for the mission. It will be very terrible if all your other soldiers are Rookies or Squaddies.
    ■ Make use of the environment. Cars, trucks and UFO Power Sources blow up when you hit them with explosives. Blow them up when enemies are near.
    ■ Always check whether the Pistol does a better hit than the primary weapon. Changing to Pistol and back again to primary weapon doesn’t cost a turn.
    ■ Don’t forget to equip wounded soldiers with default items so that their equipment can be used by other soldiers that are going on a mission. Equipments are costly to manufacture.
    ■ Council mission maps have all Thin Men. Titan Armor is quite useful to become immune to their poison attacks.
    ■ Terror site missions mostly have Chryssalids as the main enemy. These missions usually have an artificial time limit since the enemies will keep killing citizens. One of the worst things in such missions is that the Chryssalid will turn a citizen into a zombie which can turn into another Chryssalid if not killed in a few turns. One thing you can do is that use a grenade to kill that citizen before getting infected if you cannot save the citizen.

[9.0] Conclusion
That pretty much covers the major aspects of beating the game at impossible difficulty. It’s quite possible to beat the impossible difficulty with a bit of patience and forward thinking. To find out in-depth details of game mechanics you can head over to xcom.wikia.com . Also check out PCGamer's article on how XCOM player Zemalf beat Enemy Within on Impossible Ironman

Good luck. Have fun.
- chimera201