Ringleader

For this trophy, you'll need to play as Ironheade in a multiplayer match against Ironheade. Once the match starts, upgrade your own stage once and spawn a single Fire Baron unit. Once they're ready and the opponent has 40 Headbangers and/or Razor Girls (you'll want as many on the map as possible for this one) clumped together, double team with a Fire Baron, drive over there, and begin trying to encircle a large group of enemy units with a trail of fire.

The difficulty here comes from the Fire Barons not turning as easily as they do during the Metal Beast capture mission in the main story, or at least not having as much flat ground to do so - it can take several tries to get this one, and it's hard to keep track of just how many units you've caught in a single ring of fire, so keep trying till it pops.

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, using a boosting partner instead of the AI is highly, HIGHLY recommended due to the not-great controls of the Fire Barons making the completion of a ring of fire difficult even before enemy movement is factored in.