On the mission Science Center Exterior, at the beginning of the mission you will start with a sniper rifle. There are three guards that you have to snipe, one right above the front door, one on the right and one on the left of the building. Once you kill them, three more will appear on the roof of the building, one above each location from before. You do not have to get a head shot on each enemy but you just have to make sure you don’t miss any of them. If you do miss one of them, just pause the game, go down to restart and then start at the beginning of the level.