It seems to me that AI can shoot down helicopters better than human players ? I don't know if I have played enough games to make this statement but so far it seems when I shoot helicopters they miss but the AI will kill them with very little problem.
I played the Battle of Hoa Hoi 2 PBEM, and lost a disgusting amount of helicopters. I think it's just one of those things that is always against you, no matter who or what side you play. ;)
You really need to keep the helicopters out of harms way... The AI does a decent job of keeping them back from known threats. Humans tend not to know the ranges of weapons off the top of their heads. One area that the AI can beat a human in.