easy place to get this is in chapter five once you get into city hall from the back if you ran away from the monster you will end up in a tunnel make sure you have the ambush skill unlocked and take an immediate right and hang back to watch the enemy pattern when you see him walk down the hallway in front of you and disappear go into the hallway and hide behind the wall to your left. now just wait till he works his way back and once he starts to turn the corner hit x for the kill boom trophy earned. if you miss this there is also another guy that is kinda hiding in a corner so just in case you mess up the first time