Painkiller

For this trophy, you'll need to play as Ironheade in a multiplayer match against, well...Ironheade. Once the match starts, upgrade your own stage three times and summon the Rock Crusher. When the opponent has massed 40 Headbangers and/or Razor Girls, drive on over there and start, well, grinding; the only kills that count are the ones you get from running over enemies and not killing them with fireworks or the double team, so this can take a while, especially since the Rock Crusher's fireworks are more liable to kill enemy units than the runover damage is! Just be patient for this one.

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.

For this specific trophy, since it requires a hefty and time-consuming investment of fully upgrading your stage first, if you're playing against the AI you'll want to do so on the Battered Bluffs map, since it's sufficiently big enough to prevent the AI from reaching your stage and attacking you before you're ready.