10 minutes into the game, during Mission 2: Second Chance, you will receive your first and best opportunity at this achievement.
Upon climbing over the wall in the video, a van will drive by and a scripted conversation and scenario will being. Cloak, and sneak up behind the enemies, making sure to line up their heads. Shoot, and the achievement is yours.
If you miss, or get unlucky and don't unlock it, don't worry. Just use the same tactic in a different spot, as it's not worth replaying the mission from the start for this achievement.