This can be achieved on Mission 4 (Pier). You will find your first sniper rifle on the previous mission (Fort). During the Pier mission, when you're driving the jeep down the road you will soon encounter a road block with a large truck, a jeep and a few enemies. Kill the enemies and run back where you came from a little so that you can see a watch tower in the distance which is far ahead of the road block. You should go prone so the sniper doesn't sway and decrease your accuracy. At this point kill the enemy at the top of the watch tower as the tower is little above 500m away.

See video for further help: