You can get this one in mission 9, Hometown. Towards the end of the mission, you’ll be escaping from the town and will come across a campsite style area with 2 enemies. The simplest way of getting this trophy is to press Up to call Hadir’s phone distracting the 2 Soldiers. Now run into the middle of the area and pick up the revolver. If you’re to slow to get it before being spotted, simply reload your last checkpoint and try again. Aim down the sight, line up the 2 soldiers, fire one round aiming for head / neck shots. Providing you kill them both with 1 bullet your achievement will unlock.