The easiest way (if you got bots around you) is to jump in a Snowspeeder and press F2 to command a bot to be your gunner. Then, just fly slowly around the AT-AT, stay close so it can't blast you with its cannons, and wait until the bot gets the harpoon attached. Then when the camera backs away it's easy to make the rounds and take that AT-AT down.
Shouldn't this be in the Strategy forum, BTW?