Not in Kishimoto manga, Nagato had that crowd long before Itachi was born and died after Itachi so Itachi never got it. It took KM Naruto (1/13th the power tho), 8 tails Bee, and Edo Itachi to fight a controlled Nagato with low mobility while KM Naruto with 1/13th the power was fighting evenly with Itachi in close combat and Bee fighting Itachi just fine himself.
Which was used on Obito and didn't kill him...
Too bad Itachi stated it's not a jutsu that could be use in battle and we saw that by him receiving fatal blows from Kabuto. Also Obito knows all Uchiha jutsus thanks to Madara training him so he would know how to break out of it or even prevent it from being completed by changing how he reacts since Itachi have to create the same actions over again and alive Itachi can't push himself due to his illness and the fact that he was only alive due to medicine.
Too bad Itachi have no knowledge how Kamui works and even Sharingan users like Kakashi and Sasuke who are both smart, couldn't figure out how it worked on their first encounter with Obito. Only Minato ever solely figured it out with no intel in battle while Fu and Torune had intel about Obito being able to go intangible from Kakashi and Yamato's report. Obito have faster reaction speed than Itachi could attack as shown when he fights multiple opponents. Also Obito will see thru Itachi's clones since it's the Byakugan that can't see through clones while the Sharingan can see through ninjutsus, taijutsus, and genjutsus.
What if Obito used his Fire jutsu which is on Madara level and much stronger than what Sasuke and Itachi ever showcased? Itachi would have to use Amaterasu but that cause him pain, makes him more blind and lose a lot of chakra. Genjutsus won't work, Obito with just 3 tomoes controlled Kurama when Sasuke and Itachi said they need the MS for such a feat so Obito's mastery of his Sharingan is enough to see and break any genjutsu Itachi has especially since he also have his own MS and have many more years of experiences using it.
No he doesn't haha Obito got Minato, the fastest ninja in the manga, and Minato only got away by FTG. Once Obito have his hands on a target, he will absorb them and the only ones who prevented this are Minato with FTG, Konan by having full intel on Kamui and have preparations beforehand, and when he fought more than one opponents who could attack as Obito is about to absorb with intel as well. Minato said Kamui was better than his FTG which was only seconds faster than Kamui when they first fought so no Itachi's speed isn't close to being able to not get caught; he would need speed equal to or greater than A's who would have the speed to prevent being absorbed.
Sharingan will see which one is real and nothing stopping him from making clones himself or using his fire jutsu which is on Madara level and much larger and stronger than any normal katon Itachi or Sasuke ever used
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.And when Itachi breaks out of that genjutsu, the real fight will begin and he will most likely lose.
That was still a 4 on 1 fight which means Obito's attention couldn't really be solely on one person since he would need to make sure no one is attacking from different angles and Itachi have no intel on Kamui at all so that wouldn't be an issue. Sasuke couldn't figure out how Kamui worked when he tried using chidori on him, Kakashi with team Kakashi and team Yamato (Kurenia's team) couldn't figure out Kamui with his Sharingan and they also had a Byakugan user as well and was attacking Obito as a team. Kakashi only figured out how Kamui worked when he Kamui his kunai so it didn't hit Naruto and it ends up hitting Obito in the other dimension. So 2 Sharingan users who are smart and analyzers couldn't figure out Kamui with their brain and eyes alone, only Minato ever did that and Minato's smarts is highly hyped by those who knew him.
Itachi on the other hand can't push his body since his illness will kill him; he didn't receive much heavy blows from Sasuke at all really, just a stab wound on his leg really but Itachi's body started to fail him in the fight due to Itachi using his MS jutsus which does physical damage or pain on him. Susanoo being the most painful as Sasuke said it ached every cell in his body and wondered how Itachi much have felt. It also uses both eyes which make the eyes blind faster. There's a reason Itachi relies on Genjutsus; he's a pacifist and can't handle full on battles with his illness.
Thing is the Sharingan can't tell when Obito is using his Kamui since Sasuke and Kakashi couldn't tell how Kamui worked or when Obito was using it, Hinata couldn't even do this and Byakugans have better insight than the Sharingan. Itachi does not have weapons that Gai had in that scan, Gai had weapons that extended his reach and used it to help him keep his body a good distant away and to make sure Obito didn't grab him when he tried kicking him. The only weapon Itachi ever shown is shurikens and Kunais while Obito can go thru Itachi's weapons, shoot giant shurikens from his Kamui and use those as his own weapons.
Itachi lack knowledge on how Kamui works so wouldn't be doing what Gai was doing either. Itachi usually use Genjutsus but those won't work here so using ninjutsus and taijutsus is what he would do; he wouldn't start off using a MS jutsu since the side effects are too great (pain, more blind, cost lot of chakra). All it would take is for them to go into a taijutsu fight and Obito just need to grab him when blocking or attacking and it's over. Obito can even cause Itachi to use Amaterasu by using his fire jutsu which is above Itachi's normal fire jutsus
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as Sasuke did when his fire jutsu overwhelmed Itachi's. Itachi then had to close his eyes afterwards since Amaterasu is casted where Itachi is looking and it cause strain on his eye.If Itachi use Susanoo, Obito literally have to do nothing but defend while Susanoo kills Itachi, Susanoo is what killed Itachi in the manga after Itachi used 1 Tsukuyomi, 2 Amaterasu, 1 Susanoo. Susanoo is stated to ache ever cell in the body (Sasuke stated this) so the longer Itachi is in Susanoo, the more damage he will get and his body is already weak which is why he needed medicine just to prolong his life as Obito stated. Obito knows about Itachi's illness and will use that against Itachi and can just push Itachi into using Susanoo and/or Amaterasu. Obito has the chakra, stamina, and power to do this while Itachi doesn't have the chakra or health to last long.
Then how did Kakashi know Itachi's clone was a clone in part 1? Itachi even said he noticed Kakashi's clone too late in Shippuden when they fought. Sharingans can see through Ninjutsus, taijutsus, and Genjutsus so yes it can see through them. It's the Byakugan which can't distinguish clones since the chakra is split evenly amongst the clones and original.
