This is a bronze achievement worth ten gamerscore

To get this achievement you must kill 10 enemies with the Mobile Turret missiles in mission four fission. In the very beginning of this mission after you exit the helicopter there is a mobile turret on the left side of the road. It looks like a brown mini-tank with four "legs". It's standing near a hole in a wall where lots of enemies are spawning. Enter the turret and mark enemies with rockets. The rockets automatically search their targets and rain down from the sky. Kill 10 enemies with these rockets and the trophy / achievement will unlock.