Find a group of enemies (preferably a outpost) and mark them on your map, then go to 175 - 225 meters away and then equip a sniper with high accuracy (and a suppressor if you don't want to be detected) I used the Farcry 3 Blood & Dragon sniper, after this you can headshot the enemy.
If you hit the headshot you will get the achivement, if not find another group (or leave and go back to the outpost) and try again.