This is obviously easier on lower difficulties and also the further you are upgraded. I would at least recommend using the explosive or cold plasma to speed enemy deaths up/or lock them in place. The best way to do this whilst playing casually, is to take your time and try to reflect/jump over any incoming gunfire. Fighting one enemy at a time certainly makes this easier. I personally got this on one of the games many long corridors that house 5-6 enemies at a time. I went through carefully, freezing enemies and reflecting bullets whilst killing them one by one until all twenty were dead.

For an easy method, you can earn this right at the start of the game. After getting the slide ability, you can exit the room, kill four enemies then return to the upgrade point. When you exit, the enemies will have repsawned and you can repeat until you have unlocked your achievement.

I've included a video I made showing you how and where to use this method.