You actually don\'t even need to bring Wicket into the picture - that giant snow beast on Hoth was just 4 Ewoks cleverly disguised. Its a good thing the Endor Ewoks didn\'t find out what Luke did to their cousins.
Jawas are actually Sand Ewoks.
Ugnaughts are just Ewoks that have been corrupted by the Emperor, who is actually Saruman in disguise.
Which brings us back to the original thread point - AT-ATs are Imperial constructs, therefore made by Saruman. Since Saruman was ultimately killed by Grima Wormtongue, Grima can clearly take out any AT-AT.
Plus, if Grima were ever killed, he would come back as a demonic Chucky doll, and we all know they are impossible to destroy.