The more I see the Juubi the more I believe that it wasn't just the Sage in combat with the Juubi. If we go by what makes sense the Rikudou was a Sage when he fought the Juubi he did not have the Rinnegan or any super juubi defeating powers.
I would be willing to bet that the Rikudou was a master of Senjutsu and that he knew far more about it than the toads slugs or snakes. The only way I can see him being able to at least keep up is if he could use Senjutsu with no limit, ie he could gather it while moving. Tbh I think the nature of gathering senjutsu is a state of mind rather then the body. Staying still just helps you reach that lvl of peace. That would explain some what how the heck the Sage was able to seal the creature and balance it with his chakra, he could use Senjutsu all the time to balance and hold back the Juubi. He isnt like Naruto at all who probably has larger chakra reserves then him by virtue of being a Jinchūriki since birth. The Sage made this a moot point by gathering an equally large amount of Senjutsu as the Juubi, normally he wouldn't have been capable of this feat because it would over balance his own reserves killing him, but because he has the juubi he can balance these two chakras against each other. In a really weird way he himself became a Bijuu by sealing the juubi.
But back to another point I was wanting to make, I doubt the Sage was alone when he fought the juubi theres just no way he would be powerful enough, hes a human sure a really powerful one but still human. What the Sage became after the sealing is decidedly more than human. Perhaps there was a similar situation as is happening now, juubi vs everyone? The sage was perhaps a leader or perhaps like naruto just one of many trying to stop the creature. Come on if there was a creature like that walking around capable of blowing up stuff half a world away wouldn't it be in everyone's interest to stop it? There must have been a unified alliance of some sort between all the peoples of that era to try and stop it.
In all honesty I think naruto will have to discover how to do this, either that or there will be some sort of super tailed beast mode combining all the chakra apart from shikaku, it would probably only work for minutes, a one time deal super power up.
I would be willing to bet that the Rikudou was a master of Senjutsu and that he knew far more about it than the toads slugs or snakes. The only way I can see him being able to at least keep up is if he could use Senjutsu with no limit, ie he could gather it while moving. Tbh I think the nature of gathering senjutsu is a state of mind rather then the body. Staying still just helps you reach that lvl of peace. That would explain some what how the heck the Sage was able to seal the creature and balance it with his chakra, he could use Senjutsu all the time to balance and hold back the Juubi. He isnt like Naruto at all who probably has larger chakra reserves then him by virtue of being a Jinchūriki since birth. The Sage made this a moot point by gathering an equally large amount of Senjutsu as the Juubi, normally he wouldn't have been capable of this feat because it would over balance his own reserves killing him, but because he has the juubi he can balance these two chakras against each other. In a really weird way he himself became a Bijuu by sealing the juubi.
But back to another point I was wanting to make, I doubt the Sage was alone when he fought the juubi theres just no way he would be powerful enough, hes a human sure a really powerful one but still human. What the Sage became after the sealing is decidedly more than human. Perhaps there was a similar situation as is happening now, juubi vs everyone? The sage was perhaps a leader or perhaps like naruto just one of many trying to stop the creature. Come on if there was a creature like that walking around capable of blowing up stuff half a world away wouldn't it be in everyone's interest to stop it? There must have been a unified alliance of some sort between all the peoples of that era to try and stop it.
In all honesty I think naruto will have to discover how to do this, either that or there will be some sort of super tailed beast mode combining all the chakra apart from shikaku, it would probably only work for minutes, a one time deal super power up.