In the chapter Highway 17, you will come acrossed a magnetic crane that allows you to pick up your buggy to continue the story. Alternatively, you can also pick up the shipment containers that are scattered around the dock area. Just pick up a container, and drop it on a soldier to kill him. You can alternatively swing the container at the Combine Soldier, which will send him flying, killing him. Get 3 kills using the shipment container, and the achievement will unlock.