Place Mines on a snowspeeder. When you run out refill your ammo with a gonk droid and place more, then get in and fly towards an AT-AT and crash into it. The mines and the ship will go off and nearly destroy the AT.
NOTE: Be quick with placing the mines because after awhile they will disapear.
Also my 7 year old brother and I always make a supplies base somewhere (usualy ontop of the Outpost or the Bunker) and keep Taun Tauns and Snowspeeders there so later if we guard a command post and want to leave we just walk over to a Snowspeeder.
NOTE: Just making this base usualy takes up most of the game time.