They don't necessarily NEED power ups.
Kishi probably knows it'd be cheating.
Final of the manga, sudden power up, win. It's cheap logic.
He probably thought of a strategy to give Naruto(because, honestly, he's the main character), then that music(the one that always plays when Naruto is about to win something) will play and everyone will be happy. The plan will probably involve Sasuke, because this manga IS about friendship...
One possible strategy is to waste Obito's chakra. I think the relation Obito/Juubi is like how Naruto/Kyuubi was. Obito and the Juubi are pulling the chakra of each other.
Another is to hit him with the stick that blows natural energy of one's body

Kidding.
You may think that there's no weakness to Obito, but you'll see that Kishi will create one.