How many troops and armor did you bring down? Where did you put them before the AT-AT started moving? Where did you send them?
1. As much as you can.
2. On the hill before entering the plains.
3. At the crossroads where the AT-AT will pass the lake road.
This module takes plenty of patience, though, if it were reality, you wouldn't have the luxury. That's why we play games instead of doing this for real.
If I were you, I'd have about 10 AT-AA's at the top of the cliff and just spend half an hour shooting down Y-Wings. Imagine the points you'll get out of that. Occasionally send down a stormtrooper to spot the turrets around the bridge and use the heavy artillery to take them out. When you've finally taken them out and the Y-Wings start slowing down, send a squad of stormtroopers to scout out the area where you will need to charge to.
Once you've done all that, send all of your weapons to that crossroads. You can pretty much ignore all Rebel forces until you get there. Set up the artillery early enough to have any effect; use them to destroy the other rebel forces until the AT-AT shows up.
I also tried someone's tactic of sending my stormtroopers to shoot up the AT-AT before it starts moving. This is REALLY fun! As soon as it started moving, it took some hits from its formerly-friendly forces and got destroyed right away. The only bad thing is that the surviving Imperial forces from that group don't move at all so you can't use them.