Board Thread:General Discussion/@comment-24382631-20140110234616/@comment-24185694-20140602141525

Aizen's monster form tanked Ichigo's FGT(who was a transcendent) and fully regenerated. Even Ichigo was surprised that after an attack like that, he could still heal from. Furthermore, it was pretty much implied that by that point Aizen had become immortal hence why he had to be sealed.