So the easiest way to get this is to find a small group of enemies on the ground that are standing next to the helicopter with no pilot seated inside it.
Shoot, but dont kill, one of the enemies. Every single time I do this one always runs and jumps into the helicopter.
As soon as the blades start to turn give that guy a headshot before he takes off and boom. Achievement.
I did it on one of the randomly spawning groups out in the wild, bonus with having the pilot be the bagman who always gets into the vehicle to run away, but the bases work too...