Onoki needs to make contact with Perfect Susanoo to lighten it or make it heavier, that isn't ever happening.No they outsmarted Kakashi in that instance. The argument was either they outsmarted him, or they were fast and skilled enough to get a bell through force. You can take your pick which.
So far Perfect Susanoo as the ability to slice, stab, and crush. Now Oonoki could have taken one of two measures. He could lighten to the extent that he lightened the two meteors. The crushing power of Perfect Susanoo is greatly reduced. In addition, that makes it easier to deal with as its strength and destructive power would go down with the diminished weight. That would then leave Perfect Susanoo more susceptible to the 4th raikage and the 5th hokage's strength. Additionally as we know that Tsunade is an expert with transformation jutsu, and based on the performance of Gamabunta and genin Naruto against Shukaku we can imagine that Tsunade with Katsuyu could do something similar. But you may not like that argument but we are also aware that Perfect Susanoo has nothing to harm Katsuyu whatsoever. And in it's lightened state it may well be too light to casually push Katsuyu aside as she evidently weighs a great deal.
Oonoki could also do the exact opposite. He could weigh down the susanoo with a weighted rock technique that parallels his light rock technique . That in turn would increase the destructive power of the Perfect Susanoo. He would then make the kages and gaara's sand lighter so that they can move quicker to avoid the susanoo's now arduously slow movements. From there a different strategy is used. Katsuyu is against necessary as she has the most acidic acid shown to date. As we well know, Susanoo is without a doubt not impervious to acid. And with the acid the structural integrity of Susanoo will start to fail, and Susanoo will be too slow to avoid said acid. Then Tsunade could boost Mei's Lava based jutsu to work after the acid to further melt the slow Susanoo. As icing on the cake we are also aware that Jinton has some effect on the susanoo. And As Madara can only absorbs what comes to him, and he's a speck in comparison to susanoo it's possible to further take out the structure of susanoo. At this point you return Tsunade to her normal weight and have her right Gaara's sand. While Oonoki rides A's back and the beat around the greatly damaged Susanoo. All the while serving as a distraction for Gaara to gather enough Sand from the surrounding desert to create a sealing jutsu large enough to encompass the entire Perfect Susanoo. The Perfect Susanoo will be too slow to avoid. And As it engulfs the PS, Oonoki use his great weighted rock technique to weigh the sand down making it even more difficult, even impossible for Madara and his Susanoo to struggle against it.
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If you think Onoki is getting to PS before it swings it sword, then you have another thing coming, Madara could have killed them with that sword slash if he didn't try and flaunt its power. The shock wave alone destroyed two mountains. Onoki won't make it PS to do anything to it before its sword slash annihilates them.