To start, a revenge kill is when you kill an opponent that killed you last. So for example if you die to "BlazeIt69" if you kill him in your next life you will get a revenge kill. You will need 25 of these.
The best advice I can give you on this is try to recognize character skins, their out-of-match skins are the same in match. So if you can remember either their name or which skin they were using and focus on them it may be easier to find and get your revenge.