This is most easily boosted by yourself using a hangar and garage. Any location will work, but I would recommend the Landsdowne Airstrip
in the Whitetail Mountains
, as the hangar and garage are directly next to each, and you automatically unlock a plane upon discovering it.
First, at the hangar spawn a plane with Bombs like the Pack Hunter
or The Wild Goose
. Carpets bombing planes do not count.
Now, take this plane and park it close to a garage. At the garage, spawn ATVs and park them as close of possible to the plane but avoid the propellers. Once the plane has 4 vehicles next to it, get inside, Press LR + RT to drop a bomb and destroy the ATVs, and the plane, unlocking the achievement. NOTE:
If you're having issues with things despawning, try assigning a different Gun for Hire to your roster.