At his current state, how could Madara possibly be defeated?
Any physical attack/Taijutsu would be easily deflected by either:
- The Susano'o
- The Gunbai (war fan, which proved to easily negate a Bijuudama)
- Shinra Tensei (which wasn't seen yet, but we have to assume he's capable of it).
And I'm not even starting to talk about the Rikudo mode.
Any non-physical attack would be easily absorbed by the Preta Rinnegan path. There aren't
many ultra-powerful genjutsu ninja who can hope to trap him in a genjutsu for more than 2 seconds. And in the offshoot that something does hurt him, he'd just regenerate. He's a zombie.
What weakness does Madara have that could be exploited to harm him? They'd have to damage him badly to hope for a seal.