Use power limit, don't undervolt. When doing the latter, there's no obvious signs of instability like when you go too high on the core or memory clocks. You have to REALLY look for it.
1)Afterburner has an OC Scanner option. I believe Precision and Gpu Tweak have the same program. Open AB, click OC Scanner, then click Scan and wait for it to finish. [Took around 35mins for me the last few times I ran it.]
2)When the Scanner is finished, click Apply on the main HUD, then save the profile to one of the numbered buttons(for quick access). After, you click the reset button, then click Revert in OC Scanner.
3)Run benchmark/game using gpu's default profile. Record, or write down what it gets.
4)Apply the saved profile from the Scanner, click the unlink button(chain link icon), enter a lower value in the Power Limit box, and click Apply.
[This step will need to be repeated until you match your card's performance at default. Use intervals of 5 and 10%, to save some time. For my 1080Ti, I found it at 80% - this doesn't mean it's the same for your card, but it might be a start.]
5)Once you've found a match, save it to a numbered profile. Don't forget to click the Reset button when finished. Done.
A note by Steve in the pinned comment below the video:
"The built-in benchmark is more GPU bound than the actual multiplayer experience (for the most part), so while acceptable for testing GPU performance with a 13900K, I wouldn’t use the built-in benchmark for CPU testing (which makes CPU testing the game accurately mission impossible, at least for comparison with other CPUs)."
Due to how matches are always different in games like this, cpu testing is pretty much impossible, except for those on ancient cpus.