Buy at any weapon shop you want the 'AR-CL' sniper rifle with the 'Marksman ADV-X Scope'. The range with this scope is 8 of 10. If you have the gold edition from the game equip 'MBP .50' with 'Long Range Scope'. With this combination the Range is on 10/10.
Go to any outpost / location where are enemies located and set a marker to see the distance. (You need 150 meters). Mark an enemy and proof the distance again. Aim directly on the head and shot. If you miss the shot fast travel to a nearby location and come back.