Phantom Doctrine

To earn this you have to clear a mission without killing (using "Takedown" skill on civilians is forbidden even if it's non-lethal). Killing mission targets also counts, so it's impossible to unlock this achivement in mission with killing as a main objective.

I've unlocked it by completing mission type "Informer rescue". In this kind of mission as a secondary objective is always killing an enemy agent, so from the start you know where exacly is he/she, so if you have disguise is super easy, because you can simply avoid this place. If it's immposible to release informer without clearing the room form guards or civilans, so you have to look for a chance in next mission, or reload mission once aggain form the World map (of course you have to load game from last save first).

Some of story missions are also suitable for this achivement, like when you recover the Marquis cargo, but when you're playing on Hard difficulty, missions have more guards so it takes more time to solve guards/civilans moving pattern.