The description of this achievement is slightly misleading. You need not use the stasis skill on 50 unique enemies, you must merely use the stasis skill on an enemy target 50 times. I personally finished this on the chapter 5 miniboss fight, with the regenerating necromorph in the cryogenics chamber.
You are placed in a tiny room with the big baddie and three of the tentacle-sprouting infants. After doing away with the ankle biters, all that remains in this room is you, the lumbering miniboss, and a stasis recharge station. You'll have it done in no time, and for free, no less.