This is the mission where you will be providing close air support via an AC-130U Spooky aerial gunship while Ghosts go in to rescue Col. Crenshaw.
Your weapons on board the Spooky that are at your disposal are the 105 mm M102 howitzer, the 40mm Bofors L/60 cannon, and the 25 mm GAU-12/U Equaliser Gatling gun.
Switch up between these weapons as needed to take out enemy vehicles and tanks that will be coming after the rescue squad. Use the Howitzer to take out large ships and buildings, and the Bofors cannon to take out large groups of vehicles or troops by the roadblocks. Use the Gatling gun for the helicopters and everything else.
Avoid friendly fire as much as possible. Your focus is to keep the rescue squad alive until they transport him to the USS Marshall. This is a story-based achievement and can't be missed.