When Muu sees Itachi make clones, he will go behind a stone pillar (for protection from Amaterasu) and use weighted-boulder jutsu on the ground to immobilize Itachi and his clones.
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-Onoki lightened the sand underneath Madara's Susanoo simply by
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, which was in contact with Gaara, who was in contact with the ground.
That should prove direct contact isn't needed for the weighted boulder jutsu to take effect. If you want to claim that Muu doesn't have the weighted boulder jutsu, how do you think he makes himself heavy again after he lightens himself?
With Itachi and his clones immobilized, Muu just needs to finish them all off with a large sweeping Jinton. He can prepare Jinton from behind a
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so if Itachi uses Amaterasu, it will just hit the rock instead of hitting Muu.
When Itachi sees Muu preparing Jinton, he can't dodge (due to being weighted) so he'll manifest V3 Susanoo and use Yasaka Magatamas to interrupt Jinton prep. This will work once but only once, because Muu will just
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. Muu will know there are Magatamas coming at him because he's a sensor, even if he can't see past the rock he's behind. Now, all Muu has to do is use weighted boulder on the ground again, and Itachi's Susanoo becomes immobilized.
The battle ends with Muu firing another Jinton and Itachi being able to do nothing about it, since his Susanoo can't throw magatamas when it's weighted down and too heavy to move.