Scarab. AT-AT's 2 major weak points are heavily exposed- it's cockpit or even just its front legs would be an easy target...assuming the plasma can melt through the metal, of course. Even if it took multiple shots, however, the AT-AT has to deal with that the scarab wouldn't be truely inoperable until it's core was breached, since theyd be facing each other its back is safe, which leaves simply pounding through its frint armor- most of which is not necessary to its operation anyway.