The first area you can get this is probably one of the easiest as well. It is in the Soviet Installation area after the cut scene where a guy gets his eyes mashed by the trinity bosses fingers. Once the cut scene is over, you'll start behind a crate. There are exactly 6 enemies in here; one for each bullet. I did it by shooting one of the pair directly in front of you (this means that they aren't over near the oil and therefore become easier to head shot. Take out his mate next, then either the one on your right or the one on your left. Don't move unless you have to. This means that the enemy who is on the walkway will climb down to find you making the head shot easier. Finish the trophy off by shooting Mr. no eyes....he'd probably thank you (if he could see you coming!).