It is just a minor addition to the above solution by xOBKx (which works brilliantly).
I found that moving to the left of the map gives you a good view of the oncoming helicopters.
Go prone, zoom in, aim for the windscreen of the helicopter and shoot.
Wait for your aiming sights to almost return to normal, then fire again at the windscreen.
Two shoots like this should be enough to bring down the helicopter. You don't need to hit the pilot, just the window. If the second shot doesn't work, a third will.