First, equip the AR-CL with a "Marksman Adv-X Scope". The weapon is frequently dropped by many enemies. Once it is collected, it will get added to the "Sniper Rifle" section at every store. The scope costs $1,600. Next, go to the "Sacred Skies Youth Camp" outpost in Faith's region. Approach it from uphill. You can airdrop to the nearest fast travel point, "Chan Residence". Use the binoculars, and press R3 to put a distance marker on an enemy (ideally the one of the roof). Then, crouch and zoom in with the sniper scope and perform a headshot. Make sure the distance marker is more than 150m away (160+ meters to be safe). The good thing about this camp is if you miss your shot, one enemy will get behind the mounted turret. He will be standing perfectly still behind the turret, making him an easy target. If you fail (for example, kill too many enemies), air drop to the nearest fast travel point to make the enemies respawn. If you have already captured this outpost, you can reset it by going to the pause menu and clicking "Outpost Master", but you must have captured all other outposts for it to work. Note: You can also get the "Ghost Kill" achievement by shooting helicopter and plane pilots with a mounted turret (MG).