Kill sleeping enemy in face down rappel section(10 G)

Mission 7: Federation Day (2nd Rappel)

During your 2nd rappel section, where you scale the side of the building and kill two enemies on a balcony, you'll want to keep your eyes open for a sleeping enemy inside one of the windows directly in front of you. He will be inside the building about 1-2 stories lower than the balcony.
As soon as you shoot him the achievement unlocks.