1- Minato knows how to fight Sharingan.
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2- Minato uses a clone and genjutsu becomes useless. FTG speed is untraceable. During a FTG, Minato can be wise enough to leave a clone when the original would hide somewhere. From there any genjutsu attempt would be useless unfortunately for Itachi and because of Minato's superior reflexive speed, shunshin and FTG that never disappears, either he maintains Susanoo, or he gets killed sooner or later
3- Minato can also summon Gamabunta if he wants for partner method. And with Gamabunta's powerful taijutsu, senjutsu attacks, even Susanoo would struggle.
4- With FTG kunais, Minato's clone could mark the area and more the time will pass and more Itachi's range would be limited. Minato could simply decide to launch kunais around Itachi's Susanoo and the match would be almost over, only waiting when Itachi will either decide to stop Susanoo or would run out of chakra(or side effects of MS techniques) to finish him with FTG blitz
5- We've seen the range of FTG lately. With one kunai he was capable to warp people on different positions far away the other:
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With The Susanoo only being a Chakra shroud covering the user, Minato's FTG could strike a Susanoo user with one teleportation, like they did against Obito(the chakra orbs surrounding him never followed):
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And don't tell me FTG can't warp chakra base attacks. You've been proven wrong since a while and it's confirmed with Tobirama:
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By using the Goshun Mawashi not only the FTG user can seperate the Susanoo from its user but deliver the finishing blow at the same time(Minato can do it with a clone)
And that is if i give the knowledge advantage to Itachi. Without knowledge it wouldn't last a minute
Minato has this in the bag. Mid diff at most
If you consider that Alive Minato has BM, it's neg diff U_U