Single Player: Kill 4 enemies in a row with headshots

Another very easy trophy/achievement, I got this using the assault rifle on Easy. Can be done anywhere. I found doing it on human CELL soldiers to be the easiest. Just take your time and aim for the head. It can also be easily done at the beginning of chapter 5 where you can pick up the sniper rifle for the first time after stealth killing the guard on the ledge and there are multiple enemies waiting to get pwned