First, go to a boat shop or hangar (green icon on map) and get one of the planes with bombs (Carmina, Clusterduck, The Wild Goose). The Carmina plane is unlocked automatically from the "Air Raid" story mission in John's region (Nick is the mission giver for it). Next, fly the plane to any garage (green icon on map). Park the plane in front of the garage. Then, get four quads from the garage and place them around the plane. Make sure they touch the plane. It can be four of the same vehicle (quad works best, as other vehicles can disappear after placing more than two). Finally, get in the plane, stay in the parked position (do not move), and press LT + RT to drop the bomb to get the "Death From Above" achievement. It will blow up your plane and the four vehicles. You do not need to fly, just stay parked on the ground.