A little way into the "Bag and Drag" mission, you'll be given the option to summon air-support by throwing a smoke grenade. From the point of first using it, head down the big set of stairs and head right up another small set. It's right here at the top of these a helicopter will fly over and start dropping in some enemy infantry. Right at this point, load up another air-support grenade and throw it underneath the helicopters' location, right at the men as soon as possible. Give it a few seconds and the AC-130 will fire off a load of bullets destroying the helicopter and unlocking your achievement. This also makes a great way of removing the men that are dropped.