IAT-AT's take FOREVER to bring down. Since plex seems to be integrated into storm trooper platoons (2 plex launchers per group), I find infantry spam to be effective. I've had success using 4-4 points worth of infantry, moving it close to the AT-AT's and letting heavy damage dealing vehicles/artillery support. AT-AT's seem to attack whatever is closest to them, so while its wasting shots on my troopers in 'take cover' formation, its getting hammered by at least 6 or so plex launchers and whatever I have supporting them. Hellfire droids are always solid and have good range, floating fortress seems to have either a damage or rate of fire buff, and the jedi tanks they use (forgot the name) are good for kiting. Move them up, and as soon as they shoot a salvo of missiles scoot them back. Like I said, it takes FOREVER to take one down, but worst case you can also use bombers/orbital to take out a bunch of them if the AI is spamming AT-AT's. As far as being on the defensive against them, I usually just sell off all units/buildings on the planet and take it back. I also never keep ground heroes on frontline planets either. Space superiority > ground superiority and if your aggressive in space, you can usually take out ground units during space battles.