Note: Do not destroy all the spires before attempting to do this, or you will need to start a new game. Find a spire, and hover directly over the top of it. Just as it launches the missile, press [Blast] to shoot it down and get the "Top Gunner" achievement. It is not recommended to fly around at higher altitudes because of the unpredictable flight of the missiles, in addition to the difficulty of hitting a fast moving target.