During Mission 4: Proxy War
, at the start of the level grab the Kar98k sniper rifle resting on the crate in front of you. After regrouping, and storming the base upon the missile strike, sprint through all the enemies to spot 2 helicopters flying in around the base including one coming in for a landing. Line up the shot, hold your breath, and hit the pilot to bring the chopper down, unlocking the achievement.
Additionally, you can reload the checkpoint and grab all 3 of these achievements in the same spot: