When I'm using the AT-AT enemy rockets really shake the cabin quite a bit. The turrets also hammer pretty well, but the damage is not huge. In the end you need cooperation of a few rocket troops and turrets to take on an AT-AT. If you have rockets use a Tauntaun to get in close (out of gun range of the AT-AT)
Of course the Snow Speeder cable is the best bet.
On the recieving end, there's nothing worse than seeing an AT-AT's head turn in your direction after hitting it with a missle.