Disgorgers are the new timed/limited use superweapons. You always find them by using Atropian Keys, be it behind the blue doors or the locked chests. I've never found anything else in a locked chest. There are several types of Disgorgers and you will find one that suits your playstyle. You can stack them and have even 5 or 6 at a time - great for the boss battles - and usually one use gets almost rid of a boss health bar. My favorite one is the explosive one. Just max out the explosion and don't let go of the trigger. It fires automatically.
Kill 100 enemies cumulatively with Disgorger weapons and you will get the trophy.