In order to unlock this achievement, you need to get a cumulative sniping distance of 84.39 km, or 84,390 metres. You can see what your total distance is on the Statistics page of your Service Record at the Main Menu.
Without any spoilers, a useful place I have used is the end of Mission 4. Here there is a fantastic balance between distance and number of enemies.
Best of luck and Happy Sniping.