There is an even easier way.
Go to an event called "Raining Fire", there is a chopper next to it, get in and decline the challenge.
Fly to an event called "Death from Above", on top of that tall building is another chopper.
Get in front of the building and kill the new chopper, lower the altitude to around 20-30 feet above the street and then ascend back to the top and kill it again.
Repeat until you get 20.
It takes 2 shots with the middle missile and 1 with the heatseeker to kill it.
The heli's you kill don't have to be manned