A great way to grind this trophy out is to have you and your buddy go to The Pit in Graterford, PA. After you run into some Grims and Leeches, you'll come into a circular arena-like area where tons of Grims, Leeches, and Leapers attack you.
Both of you should equip the Cryogun and use the secondary fire to hit the enemies. As you progress through the area, make sure you're generally aiming in the same areas so you can hit the same enemies. Once you get near the end of the checkpoint, just let the remaining enemies kill you to restart the checkpoint and respawn the enemies. It will put you right outside of the arena area and all of the enemies will respawn, but your trophy progress won't be erased from dying.