I found a straightforward way to get this one. When doing a Wanted Dead mission, your marker tells you how far you are from the objective — you can use that as a range finder to gauge your distance from enemies.
Equip your bow with the Marksman Sight, then position yourself about 75 meters away from your target for safety. If possible, find an elevated position for a clearer shot.
Now, aim down at the 80-meter mark on your sight and take the shot. You should unlock the achievement on your first try!