Tobirama should interrupt Hashirama and Madara's fight by teleporting into the fight, grabbing Hashirama, and teleporting him back to the others. He should then explain to Hashirama that only him and Naruto can hurt Obito, and that he, Sasuke, and Minato will fight Madara in place of Hashirama.
So that way you'd have Naruto & Hashirama teaming up against Obito while Tobirama, Sasuke, and Minato team up against Madara. (With Hiruzen probably jumping in soon if Juubito didn't destroy him)
So that way you'd have Naruto & Hashirama teaming up against Obito while Tobirama, Sasuke, and Minato team up against Madara. (With Hiruzen probably jumping in soon if Juubito didn't destroy him)