Explodemon

The absolute best place for this is found in the tunnel right before the goal room in 2-3. You need to break the three barriers along the way and try not to kill any of the enemies. I used the aim explosion to knock boxes out of the way and then waited until all the enemies were at the other end to explode the second and third. You then need to jump down into the goal room and come back up. At this point the spawn generator should spit out more enemies. Go back to the right and come back to this tunnel again. It should be filled with enemies and an explosion run back and forth will net you the trophy.

Thanks to Ultra Mantis for the tip.