Early in Chapter 3 you will exit the car and have to clear the area. In front of you and 2 the left are 2 large buildings connected by what appears to be a zipline. On the roof of the far building (which can be accessed from the back of the building) is a Marksman Rifle leaning on the wall next to a window you can use to kill the commander.
After clearing this area you return to the car and move forward, but then need to enter the bunker on the left. Equip your Marksman Rifle and once the door opens the old lady will kill the guard who comes out, announcing your presence. Hold L2 to look down the scope and strafe around the corner. There are 4 troops in this first room (with potentially 2 more who will run into the room). Shoot 3 without leaving scoped view to earn the perk.
Note: This is easiest done on the lowest difficulty setting.