Play through a single player mission in Ghost Town under any difficulty. As you progress through the level there are several opportunities for you to pick up the laser designator. When you get to the last part of the mission, where you face the Legionnaire in his helicopter, use the laser designator on his helicopter. While the legionnaire is shooting at you, the helicopter is for the most part stationary. If you time it correctly, you should be able to able to guide the bomb and hit the helicopter. The easiest way to do this is to start locking onto the helicopter immediately after he shoots a few rounds at you and starts to fly around to get into another position.