In the Destroy the Anti-Air Emplacements Side Op, rescued and extracted all prisoners via chopper

To unlock it, begin the mission Eliminate the Anti-Air Emplacements on Normal difficulty.

There are a total of 4 prisoners that need to be rescued in order to unlock the achievement/trophy. The first 3 are found in the camp near the spawn. Just neutralize the enemies, and carry them down to the chopper landing spot.

The final prisoner is a little more difficult to attain, but can be found inside the holding cell where Paz was found in the main mission of the game. Plant C4 on the Anti-Air establishments, and then grab the prisoner and make your way to the exit while blowing up the Anti-Air units as well as the armoured vehicle that shows up afterwards.

The chopper will land at the extraction zone, and youll need to load all 4 prisoners (alive) into the chopper, to gain this award.