Very Easy Gunning Achievement
While trying to get this achievement, on my first try. I discovered a sorta glitch that makes this achievement beyond easy.
What you need to do is do something that will get a strike squad called in on you, When the strike squad comes in Hijack a Helicopter.
Take out the rest of the Strike squad and any remaining helicopters that are gunning for you.
Now ascend/fly up as far as you can go and stay that high.
If a helicopter spawns while you are ascending, while at your full ascent, take it out or wait for it to fly out of range and disappear.
Now when helicopters spawn, a red icon with the picture of a helicopter and an arrow will appear on the screen telling you where an enemy helicopter is.
But because you are so high up, when they spawn, They spawn frozen in the air, they dont move around, and they dont attack.
Just target the chopper and fly close enough into range to take it out and then wait for the next one to spawn and take it out. Rinse/repeat.
Sometimes when the little report that tells you the value of you damage and casualties comes up it takes a few seconds for the next chopper to spawn, Just wait it out and you will have 20 helicopters in no time, like shooting fish in a barrel.
I reset my game just to make sure this was not something that only effected my current session and it still worked.