Sniper Elite 3

This achievement must be done on Mission 4: Fort Rifugio and is quite easy to do this if you have the right path, and are playing on Cadet difficulty. Just make your way through the middle of the first area avoiding enemy contact, and head up the ladder that leads into the base on the right hand side.

From there, make your way to the informant to trigger a cutscene and unlock the achievement. The key is not needed for the achievement to unlock, but is needed to complete the level.