Crysis
40 Achievements
1,000
15-20h
No Fly Zone
Destroy 5 helicopters
25
How to unlock the No Fly Zone achievement in Crysis - Definitive Guide
Just wait until the helicopter starts to hover overhead and unload a clip into it. There should be plenty of opportunities to take them down in "Assault" but if it still hasn't popped by then there are also a few flying around in "Onslaught" and the tank cannon works on them just fine ;)
Helicopters appear randomly throughout the game. If they spot you, so will drop back or others may attack, so watch out for this. You can destroy the helicopters with almost anything, but using your gun will take up a lot of ammo. The missile launcher is your best means of getting rid of them fast and it will only take 2 rockets to down one
P.S. This one is a little glitchy, took me 9 kills before it popped.
You will see enemy helicopters by North Korean soldiers throughout various chapters later on in the game (starting on chapter 3). Simply take them out with the missle launcher weapon (it requires 2 direct hits from it). You can easily farm the Onslaught level as it is super easy to take out a helicopter from your tank. Simply shoot it down hit start and restart from checkpoint.