I originally thought it popped early for me too but then I realized I played a fair bit of multiplayer before getting knee-deep in the campaign so I can only assume that killing the player-demons in multiplayer with glory kills counted towards that trophy. I can only assume that was the case for me unless that trophy is indeed funky (better than not popping at all though). EDIT: Oops! For some reason I got mixed up with the Rip and Tear trophy, so ignore that. I think my Juicin' It Up popped normally. Silly me.