Ghost Kill Achievement Guide (150m Sniper Headshot)
First of all, buy any sniper rifle. The first one you'll unlock is the 308 Carbine (unlocks when you gain 3 levels of resistance in total over the 3 regions).
Next, head to the Drubman Marina in Faith's Region. Set a marker on the outpost and then go to the treeline next to the water just south of the outpost. From here, there are two enemies on the dock you can see. Simply headshot one and the trophy will unlock shortly afterwards.
Note: If you miss, fast travel and come back to reset the enemy positions. If you have also cleared every outpost, you can reset them all from the pause menu.