Rockets are your best defense. I did shoot 1 or 2 down with machine guns. Went through a lot of ammo and discovered that standard ammo was the least effective. Toggle for incendiary rounds. Look for and save your rockets until you really need them. I also shot down some while I was in the tank. Secondary fire with the grenade launcher works if you get it right. Heavy fixed machine guns work but you're stuck in one place and you can't get the angles right most of the time. Also the ones on the back of the HumVee's.