I wouldn't advise you to use the machinegun on the roof because it has a smaller line of sight than the distance the helicopter can reach to. If you are out of RPG, out of Rifle Grenades in Bradley's M4, then you should use the dragunov, or foley's sniper rifle. If you don't have a sniper rifle, you can get one by following the path going below-earth on the last part. There you'll find the dragunov sniper rifle, take it with foley, and fire two magazines on the helicopter. It is very effective, and if you aim well, it should kill the choper long before the two magazines are out.