In the mission Strongohold, once you get to the top of the Church, you will have to cover Zussman from enemy fire and you can't miss a single shot.
-Press L3 to enter focus mode to be sure you don't fail
-If you fail the shot, just restart the checkpoint
-If someone kills your target first just before you shoot it, just restart checkpoint
-You don't need to kill all enemies
You can see the video below to help you through the mission. Its not my video.