You need to get this achievement on the mission "Stronghold" which is the 3rd mission of the game. I would recommend doing this on the easiest difficulty.
Near the end of the mission you are required to climb the tower and snipe enemies to protect Zussman. From here you need to not miss a shot, the best way to do this is to hold down
which puts you into "Focus mode", which slows everything down for a short time, allowing you to get that shot.
Take your time with each shot and if you miss, restart at the last checkpoint.
For any who prefer a visual guide, I've made a video for this acheivement: