This can be done during the third enemy encounter in Mission 5: The Storm. There will be one enemy sniper in the area (Scout Trooper). Scout Troopers carry a sniper rifle and have darker armor than other enemies. You must kill the sniper with a stealth melee attack from behind. This does not work if the enemies have noticed you. Just watch where the sniper goes and hide behind cover until he stops walking. Then, sneak up to him and perform a stealth kill (press R3) to get the "Master Of Deception" trophy.