You can get this on Mission 11 Captive. Once you see the fleeing Helicopter its time to start being careful. Make sure you move in between shots and stay crouched and even crawl if you have to. There should be plenty of cover spots where you can stop and deal with the enemies without having to worry about the sniper. I found that hugging the right side had the best angles to avoid the sniper even being able to get an eye on me. Make sure you are always aware of where that laser is pointing. Also you can safely move in and out of cover but i would suggest moving fast and using crouch and slide when needed. Once you get to the building door where the sniper is go in slowly and hug tight right and be aiming up to shoot him as soon as he comes into sight.