Hard trophy but with practice it's possible to get it. Needless to say you must play the mission in Regular difficulty (the easiest), in this difficulty every enemy will follow a pattern that is always the same, and most will spawn and wait for you in the same exact spots every time. Learn and study these patterns and all the spots where hostiles spawn/wait and eventually you'll get the trophy. Took me two hours of practice to get it. Advices: stay allways behind cover and crouched, and once you know the enemies spots just snipe them very carefully. Good luck!