I am having a difficult time downing the helicopter with Carlos in it. I keep running back to the supply point to get more ammo & health restored for Ramiraz & myself. After many attempts on the guns, I normally get Ramiraz killed then eventually myself. Any tips?

I was able to hold the gun on the chopter until it was damaged enough to fly away.
The chopper will then crash and you get to the next level where you have to find sanchez.