I've got this one pretty quick.
After the first mission, the Stingray Area must be liberated (5 settlements).
At the first settlement (Satellite Station: Farad) I was overwhelmed by enemy choppers.
It doesn't matter if you use the Rocket Boat's missiles (
) or minigun (
In fact, there are so many enemy choppers in this DLC that getting this achievement will be like stealing a candy from a baby!