how do you defeat gunship in crysis 2?

Answer 1

This boss?

Watch this video.

http://www.youtube.com/watch?v=75RVpd7DS6Y

Answer 2

Take out the 2 gunners, and then during the intervals when the copter temporarily retreats, quickly try and take out as many CELL troops on the rooftop/skylight as you can.
When the copter finally starts firing barrages of rockets at you, it's vulnerable to regular gunfire. If for some reason you aren't able to find a rocket launcher, there's an ammo resupply crate towards the back of the main chamber (near the entrance/exit leading to the elevator) with a pretty powerful gun lying on top of it.
It's pretty effective against the copter, although it still requires at least a dozen if not more shots, but you can always replenish your ammo at the crate. Eventually, it'll go down and you'll be able to escape.
And whatever you do, steer clear of the rocket barrage as it can kill you instantly if you're unlucky, forcing you to restart the entire fight.