Resistance 3

Once you have the Rossmore fully upgraded, its alternate fire turns into an Incendiary Grenade, which sets fire to any enemies near where the grenade hit. Once you have the Rossmore upgraded, load up the checkpoint Graterford, PA: The Pit, and continue past where you pick up the cryogun. You will drop down in to a circular chamber where several Grims and Leapers will spawn.

Allow the first six enemies to spawn, wait until they are close together, then use the Rossmore's secondary fire by pressing . This will set anything nearby on fire. If 6 enemies were set on fire, your trophy will unlock. If you didn't get all 6, simply let them kill you and try again. This can be gotten almost anywhere, but this is one of the easiest spots to do it.

Another easy location is during the Widowmaker fight in New York opposite from where you enter the battlefield, several Chimera will bunch up together. Simply sneak up behind them by walking along the left wall of the area, then hit them with the secondary fire.