You can do this in the First Chapter on the train section. When you exit that first door on the train run up to the ladders but DO NOT climb them. If you wait here down below one of the guys above will throw a grenade at you which you can pick up and throw back - Aim directly at the guy above or higher up and throw it directly at him, hitting him will detonate the grenade and most likely kill BOTH guys there. Trophy unlocks.