Kill all enemies without leaving the dance floor in the city outskirts

This is obtained in Act 2 - Chapter 1 inside the club where you will have to fight a miniboss on the dance floor. Kill him and pick up his big machine gun that he drops and stay on the dance floor without stepping off it. Many enemies will spawn down below the dance floor so kill all of them and the trophy will be yours as long as you hang back. You can also Leash enemies up towards you without risking your trophy unlocking.