If you've played this game, you know that dismembering necromorphs (also known as "enemies") is pretty much the whole game, and the best way to kill the bad guys. Remove 25 limbs and this achievement pops.
Interesting fact: If you stomp (RB) or melee (RT) dead necromorphs, those dismemberments count, too. It has to be necromorphs, though -- don't go stomping dead human corpses expecting it to count.
You can track your progress in the Start menu, under "Achievements".
Repeat 100 times and you'll get this: