This achievement will happen overtime, I was still on Yello Jello when this achievement popped for me. Like 50 hits (Monster Killer) this achievement will be a natural progression in the game.
Remember each re-try or reload of a level also counts towards this achievement.
If you wanted this achievement quickly you could load up a level 100 times and the same monster(s) 100 times over but with the amount of levels in the game it's probably not worth it.