You'll need to kill 200 cumulative enemies in Battlemode. This is easier with the Slayer because he's fighting two demon players and the demons can respawn in a round while he can't. If you're boosting this, it'll probably take around an hour and a half or two hours. It's not a terrible grind, but it won't happen overnight. Weapons like the Ballista and Chaingun are good at doing massive damage to player demons. Also, the Revenant has the least amount of health of all the demons so have the other two players play as him if you're boosting. Again, it's just easier to do this with the slayer. If you kill the slayer as a demon, this still counts.

You can track your progress with the Milestones section of the pause menu.

Trophies can be earned in private matches!