This can be got at any time during the game, I got this randomly when I wasn't aiming to get it. The earliest you can get this is during the first mission, or prologue as it's called. When you reach the main target (best done on Cadet) aim for the eye region, might take a couple of tries but it will work.
Otherwise pick any location and get close to your target, preferably someone standing still. Control your breathing using R2 and line up the shot, then boom!