The gun you need for this is the Anti-Material Rifle. This is the starting sniper rifle for Piers. An easy place to get this is at the very beginning of Chris’ Chapter 2 Campaign. As soon as you crawl out of the overturned vehicle, turn around and run back as far as possible. Equip the Anti-Material rifle, hold and press to switch to the thermal scope. Aim for the head of the enemy inside the window in the distance and the achievement should unlock once you pop it’s head. This will be easier if you happen to have the Lock-On skill. If you miss, you can restart the checkpoint to try again.