This can be done at any point of the game, or even after completion. You'll need the appropriate "Death From Above" skill in the skill tree.
Go to Dr. Earnhardt's Mansion, then go to the X: 350 & Y: 700 using the glider. At this spot, enemies will always spawn. If they are fighting your allies, just respawn and try again.
Approach the base, and use your rock distraction to have 1 of the enemies go under the zipline. Once they are in place, use the zipline, and detach above them to get the automatic death from above kill.