Very easy achievement to gain early on in the game.
Aim for a necromorph's legs if it is a walking necromorph. Once it is down on the ground, run up to it and use the RB button to stamp on it. You can also use the stasis power to slow it down should you worry about taking damage.
On enemies that already crawl, stasis and then stamp on them. This is because the crawling ones tend to grab you quicker if you go in to stamp them and it isn't worth wasting precious health on them :)
Repeat this step 25 times and you should unlock the achievement around chapter 4.