I found another way of doing this in which you have infinite missiles (could just be because my Helicopter skill was what it is so I had more ammo not really sure) and they don't fire back at all. The mission where you got to hijack the helicopter and serve as transport helping the soldiers destroy infected. The mission before the bloodtox gas and the super soldiers come. Take the helicopters and follow the mission as instructed till you have to land before the hydras appear and you have to take out like six of them or so. Since it has infinite of the first type missiles and the other helicopters won't even shoot back.
Some will spawn and attack the infected and others higher up will just spawn and fly around. They will be friendly targets since you have some soldiers inside yours you can't lock onto them so it would take a little while longer, but as long as you avoid getting hit by flying debris from the Hydras you shouldn't take any damage at all. Plus it is a great way to take out the water towers for that achievement if you haven't gotten it by then. Just look for a bunch of birds flying around and lock onto whatever you can shoot and unleash hell upon them. Once all is done you should have Gunner done and you can finish up the mission.