This achievement you can do on the first region map "Mining Town" on north in the beginning of the train station area. Before you go there, equip a 10x zoom and a bipod on your sniper rifle.

After that fast travel in the north to the train station area. There is a sniper on a tower on the left side of the entrance, mark this sniper and go 500+ meters away from him. Use you bipod, lay down on the ground, set your sniper rifle to 10x zoom, distance 400 meters and move the red dot to the head of this sniper and then pull the trigger.

Go in the settings from the game and activate aim assistance and sniper "red dot" aim helper If your still struggling.