Yeah, I had the first set of Oasis in mind. Usually if someone from our party gets on the AA gun right at the start it's enough for most pilots. If they're working in tandem with the UAV it's a little tougher but as long as we spot it before it takes out the AA we're still good. In the event that the AA gets taken out there's two mounted machine guns w/ shields and two tanks, it's not ideal, but it forces the pilot to alter his flight pattern to avoid the .50 cals.
Two be honest it's been awhile since i've faced a good pilot/gunner combo and it's usually because I just ignore them. Seriously, just take cover and hide somewhere within line-of-sight of a crate and your good.
People circle-strafing choppers doesn't bother nearly as much as when my entire team falls into their spell. It's like they hypnotize everyone into thinking priority #1 is taking out the chopper instead of, you know, protecting the crates. And what does it get you? Two tickets? And even that's not guaranteed if they bail out. Two less troops on the ground that can't arm crates sounds pretty good to me.
"But they keep killing me!! I can't even take 5 steps!!!"
HIDE you dumbshit. Use smoke. Move from cover to cover. Where the hell are you trying to go anyway? far away from our base and our crates, right? Well good, I'm glad you're dead.
One to two, at most three people should try to take the chopper out. Anymore and the risk/reward ratio is completely in their favor.