I did this as the first achievement in the game because… well, I regret nothing.
It was very first part of the mission, ‘behind enemy lines’. I was on top the tower. Using the ladder as 12 O’clock, turn to 3 O’Clock, just past a smoking chimney. There is a guard standing on the upper floor of a roofless building, just pacing back and fourth on a predictable route. I shot from the prone position between the broken bricks of the tower.
I recommend saving just before going for this so, you can reload if needed. I also recommend holding the empty lung button (RB
).