This strategy needs a massive amount of luck. Send Princess Leia up and use her tractor beam to pull down the AT-AT Walker head so that its blasters cause some damage. Send Obi-Wan down below the bottom planet and force push the far bottom right structure into oblivion. We have the better strategy.
Send Leia towards the northern rim of the smallest planet's gravitational field. Use tractor beam to pull the AT-AT planet in the southernmost planet. TIE fighter will crash this. And Leia will push the leg between the smallest planet and the largest planet to kill most of pigs. Send Obi-Wan towards the rim of the atmosphere of the southernmost planet and he'll fly to the southern hemisphere of the largest planet. Push the structure to finish the remaining pigs.
See Video Walkthrough.