Now, as up untill now we've seen Tobi rasengan'd, slashed, amaterasu'd, exploded, and bugg'd. He hasn't been hurt by ANY of it. Now, I'm not sure if Kakashi (in the manga) said this, but Tobi's probably using a dimensional-sharingan thing. He's using the same thing as Kakashi's MS, only it's obviously much more advanced and etc. Tobi/Madara can't be killed by normal means, not even through amaterasu (or, presumtively, susano).

So what if Madara faced a blast from Kakashi's MS, sending him into another dimension. I'm nearly 100% sure that would at least hurt him.

Discuss :P