Moving your AT-AT's in a tight formation does help considerably... as does using near-by hills and rocket-troops.
However, I have been able to bring down every AT-AT I have come up against... in every formation and defense pattern imaginable. I love flying between the legs of the AT-AT, also the tow cable is usually long enough to fly around two AT-AT's... however you only bring down the AT-AT that the tow cable attaches to originally.
A good strategy, along with the AT-AT formation/defense, is to take over Echo Base ASAP!!! It is rare to bring down a good pilot, but it does happen, and if you can keep him from grabbing another T-47 after it respawns, you have put the rebels in a very tough place to come back from.
Also... blow up all the vehicle repair droids. I noticed most people who fly are not pilots, and they have to land near a droid to get repairs.
A better defense forces me to become a better pilot... and that makes for a better game.