I have played this 6 times now...
Each game lasting an hour and I have still have not managed to finish it...
The problem is not in defending the boarders but rather attacking the central base The Guard Towers just nuke everything I've got...
I have landed four Coppers (IE Three Bio's Tanks, One Annihilater) and taken the city...
The problems I can't get reinforcements in fast enough and don't have enough money to support defending (right hand) country's boarders and produce reinforcements...
What tactics can you suggest...
By the way I'm play on medium and I only manage to get 4 Choppers across...

I had some difficulty with that one, until I discovered that the important thing is to kill all UNITS of the enemy - NOT the Towers!... So much easier

Here's what I did:

I took less hellis than you, but I loaded them with my best units (NOT anihilators yet, they get killed easy). I put the strong units in front of the hellicopters!... This way the hellos can come back more times
I invaded the right side of the last enemy territory, dodging his towers on the right side. This way enemy put some gerrillas. The guerrillas are lured into the territory near the mountain, wher I kill all, but he cannot recruit more because I have stuff there in the forest!
In the territory near the mountain I fly hellos back & forth with any type of stuff - you simply CANNOT loose your foot there, or he fields 5 rocket launchers! (in my case he did field those, but I killed some with my anihilators close to the mountain and landed tanks to kill the rest - you should be able to avoid this disaster).

THEN, after I had most of their units routed and had no more units to send (I already had conquered the other cities, this is important) I took his two cities WITH THE HELLOS That was really fun!