First off let me start by saying that this would be an incredibly close battle, and it is only SLIGHTLY in favor of Muu. But I do think it is, and here's why.
Itachi's arsenal
Powerful Katon Techniques
The sharingan, allowing him to copy techniques and see one's chakra
Tsukiyomi
Weaker genjutsu, not needing eye contact (uses finger)
Amaterasu
Susanoo, with the Sword of Totsuka, Magatamas, and the Yata Mirror
He uses clones that split into crows to confuse the enemy
Incredible intelligence
strength in Taijutsu and the use of kunai and shuriken
The sharingan, allowing him to copy techniques and see one's chakra
Tsukiyomi
Weaker genjutsu, not needing eye contact (uses finger)
Amaterasu
Susanoo, with the Sword of Totsuka, Magatamas, and the Yata Mirror
He uses clones that split into crows to confuse the enemy
Incredible intelligence
strength in Taijutsu and the use of kunai and shuriken
Muu's arsenal
Dust release, able to turn whatever it envelopes to dust
The ability to turn completely invisible as well as making his chakra signature disappear
Earth, wind, and fire affinities
The ability to split into two bodies to avoid an attack
Very powerful sensing abilities
The ability to alter the weight of a specific target, including himself, allowing him to fly
So to begin with, I shall state a scenario that is entirely plausible.
If these two were to start out a battle, Muu could start off by using his invisibility technique. Even though Itachi can see chakra, Muu can erase his chakra signature, so Itachi should not be able to see him. This would give Muu all the time in the world to hit a killing blow on Itachi. Even if Itachi throws up Susanoo, a massive jinton that envelopes it would certainly turn him to dust, since the Yata Mirror can only protect one part. Also, considering that 25 of Madara's Susanoos got completely turned to dust without even one escaping, Itachi's far less mobile Susanoo should have no chance.
Now lets say, for some reason, the battle commences, and this doesn't happen, and Itachi escapes, or Muu just doesn't go invisible. Muu theoretically still has foils to all of Itachi's abilities.
Dealing with Susanoo
The ability to turn completely invisible as well as making his chakra signature disappear
Earth, wind, and fire affinities
The ability to split into two bodies to avoid an attack
Very powerful sensing abilities
The ability to alter the weight of a specific target, including himself, allowing him to fly
So to begin with, I shall state a scenario that is entirely plausible.
If these two were to start out a battle, Muu could start off by using his invisibility technique. Even though Itachi can see chakra, Muu can erase his chakra signature, so Itachi should not be able to see him. This would give Muu all the time in the world to hit a killing blow on Itachi. Even if Itachi throws up Susanoo, a massive jinton that envelopes it would certainly turn him to dust, since the Yata Mirror can only protect one part. Also, considering that 25 of Madara's Susanoos got completely turned to dust without even one escaping, Itachi's far less mobile Susanoo should have no chance.
Now lets say, for some reason, the battle commences, and this doesn't happen, and Itachi escapes, or Muu just doesn't go invisible. Muu theoretically still has foils to all of Itachi's abilities.
Dealing with Susanoo
Due to his ability to fly, Muu should easily be able to just fly out of the range of Susanoo's sword.
Also with his versatility in the sky and his sensing abilities, he could dodge a Magatama.
And as I mentioned, a Jinton big enough would overcome the Yata Mirror.
Overall, Susanoo is actually the least of Muu's worries.
Also with his versatility in the sky and his sensing abilities, he could dodge a Magatama.
And as I mentioned, a Jinton big enough would overcome the Yata Mirror.
Overall, Susanoo is actually the least of Muu's worries.
Dealing with Genjutsu
In flight, Muu should be too far away for eye contact, eliminating the problem of Tsukiyomi or something lesser.
Now, if you say that eye contact IS possible, there are other solutions.
Muu, being a sensor, can easily break out of a lesser genjutsu by noticing that his chakra flow is irregular. So nothing short of Tsukiyomi would work on him.
And Secondly, using his sensing abilities, he could track Itachi in that way, meaning that he wouldn't need to make eye contact to begin with, he could just follow Itachi using his sensing skills, similar to how he dodged Naruto's attack in the same fashion.
So neither genjutsu nor Susanoo are a problem. The main problem is Amaterasu.
Now, if you say that eye contact IS possible, there are other solutions.
Muu, being a sensor, can easily break out of a lesser genjutsu by noticing that his chakra flow is irregular. So nothing short of Tsukiyomi would work on him.
And Secondly, using his sensing abilities, he could track Itachi in that way, meaning that he wouldn't need to make eye contact to begin with, he could just follow Itachi using his sensing skills, similar to how he dodged Naruto's attack in the same fashion.
So neither genjutsu nor Susanoo are a problem. The main problem is Amaterasu.
Dealing with Amaterasu
Here, the most important thing for Muu would be to use his sensing abilities to notice when Itachi is going to use Amaterasu. This way, he could prepare an earth wall in front of himself to protect himself from the flames. And being a sensor, this wouldn't hinder him because he would still know where Itachi was.
If Itachi were to hit, Muu could split, and thus avoid the flames. However, if he were forced to split at any point, the battle would shift completely over to Itachi, because Muu's chakra would have dropped greatly.
If Itachi were to hit, Muu could split, and thus avoid the flames. However, if he were forced to split at any point, the battle would shift completely over to Itachi, because Muu's chakra would have dropped greatly.
Dealing with the rest
If Itachi were to attempt to use clones, he might be able to buy some time, but in reality, it would be a bad move. It would lower his chakra immensely, overall, and Muu would have all of the same solutions to Itachi's attacks. So this would be a tactical error on Itachi's part, and so with his intelligence, he probably wouldn't revert to it.
Shuriken and Kunai obviously are stopped with an earth wall.
Itachi's regular katon can be countered by Muu's, as well as by a wall.
So overall, Muu has a way to avoid all of Itachi's attacks. And Itachi has no answer to a massive Dust Release attack. So overall, Muu would have the upper hand. However, if you take Itachi's intelligence into account, it could be very close. That is why I say Muu wins 55 out of 100 times. It would be very even matched, and a very good battle.
Shuriken and Kunai obviously are stopped with an earth wall.
Itachi's regular katon can be countered by Muu's, as well as by a wall.
So overall, Muu has a way to avoid all of Itachi's attacks. And Itachi has no answer to a massive Dust Release attack. So overall, Muu would have the upper hand. However, if you take Itachi's intelligence into account, it could be very close. That is why I say Muu wins 55 out of 100 times. It would be very even matched, and a very good battle.