Since people who don't have this achievement unlocked can't see what the requirement is:
This achievement requires you to destroy 100 Mimics, and must be done in Deathmatch mode. Infiltration and Survival Mimic kills don't count.
I recommend going for this achievement last as you'll make some progress towards it while doing the other achievements. If you need to grind it out though, a quick way to do this is to set up a game with no time limit and the max score of 50.
Move player 1 to an open, flat area. Move player 2 next to them, stand there for a while, then destroy them with player 1. Repeat the process, alternating which side of player 1 you move player 2 to. The idea is to not destroy the original immediately, that way their Mimic doesn't disappear before you can get the kill.