Dollpocalypse

For this trophy, you'll need to play as the Drowning Doom in a multiplayer match against Ironheade. Once the match starts, upgrade your own stage twice and spawn a Brood. Once the opponent has half a dozen or so units of Headbangers and/or Razor Girls spawned (you can do this one faster due to the low number requirement), double team with the Brood and drive over, pressing × before you reach the army to send the Brood careening into them, then exploding. The trophy should pop shortly afterward since 6 hits are all but guaranteed here.

As with the other four unit-specific trophies, this can be earned offline by playing against an AI, but it is much, much easier to aim for with a boosting partner who can spawn lots of Headbangers and have them simply mass and wait in front of the stage while you make your preparations and eventually attack them.