There will be two situations in the game where you must cover your allies with sniper support. The easiest level to unlock this on would be Level 7 - Gonapal Camp. During the middle of the level you will eventually come to a tower that overlooks a few other buildings. There will be several enemies ahead of you on the roof, and a few more to the left of the building. Take them out without missing a shot and the achievement will unlock.
If you happen to miss a shot, simply fire at one of your allies and the checkpoint will restart leaving you with another chance to unlock it.