So there is a lot of predicting going on how Madara survived the fight.
Some people say Madara used a clone but the only clone that we have seen so far that is able to tank so much damage is a mokuton bunshin, and at that time only Hashirama had it.
Other people say that he put some of Hashi's DNA in his wound, but Hashirama made sure that he killed Madara so that is not possible without Madara awaking from the dead or something.
Some people say it was a genjutsu, and those people are probably right. It has been shown multiple times in the manga that the genjutsu of the Uchiha are depicted like this:
Sasuke used a genjutsu just before Danzo and he had their final clash. This genjutsu was to confuse Danzo.
Itachi used this genjutsu to confuse Killer Bee.
Madara used this genjutsu to confuse Hashirama.
This is not the only proof, Hashirama thought that he saw something or someone dissapear, which must have been the real Madara:
So my guess is that Madara had placed Hashirama under a light genjutsu which is hard to notice, and then switched with a clone or something. After Hashirama left the battlefield Madara came back and got DNA from the mokuton bunshin.
Madara couldn't have been tricked by a mokuton bushin because he said that he was the only one who could see through them.
Some people say Madara used a clone but the only clone that we have seen so far that is able to tank so much damage is a mokuton bunshin, and at that time only Hashirama had it.
Other people say that he put some of Hashi's DNA in his wound, but Hashirama made sure that he killed Madara so that is not possible without Madara awaking from the dead or something.
Some people say it was a genjutsu, and those people are probably right. It has been shown multiple times in the manga that the genjutsu of the Uchiha are depicted like this:
Sasuke used a genjutsu just before Danzo and he had their final clash. This genjutsu was to confuse Danzo.
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Itachi used this genjutsu to confuse Killer Bee.
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Madara used this genjutsu to confuse Hashirama.
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This is not the only proof, Hashirama thought that he saw something or someone dissapear, which must have been the real Madara:
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So my guess is that Madara had placed Hashirama under a light genjutsu which is hard to notice, and then switched with a clone or something. After Hashirama left the battlefield Madara came back and got DNA from the mokuton bunshin.
Madara couldn't have been tricked by a mokuton bushin because he said that he was the only one who could see through them.