[Radford999]

I was trying to do the method in subway (luring a grim out of the way). That didn't seem to work. But then i went onto the pit. I froze 4 enemies at once, immeaditely posioned them with the mutator secondary fire. Then when all 4 were pukin up i used the rosemore secondary fire on them. It worked first time! I then just repeated it 3 more times and the trophy was mine.

Freeze 4 enemies at once.
Immeaditely use the mutator secondary fire to poison all 4.
When they are pukin up use the rosemore secondary fire.

Maybe it doesn't work if you do it to 3 or 5 or 6 enemies. Maybe it just works for 4. Because i was doing the exact same method for the subway level on 1 grim and it never worked.

And no, i don't think it has anything to do with the order of the trophy description. Because i froze, poisoned then burnt and i did it within 10 minutes!