I absolutely loathe Itachi, but rape thread, seriously? If Itachi loses it will be high-difference indeed. Mei is taken out in the start with Tsukuyomi. He can control the Time/Space mechanics and can leave her to rot in the corner, whilst she succumbs to physical torture sensations for 4 days mentally. She will croak in minutes. She has no speed feats or Taijutsu ones to counter his Ms genjutsu. She's basically irrelevant in this fight.
Itachi has incredible speed as we saw when he countered and effortlessly fought both Bee and BM Naruto. A is not faster than both of these. Even if he charges his Raiton Armour to the max, he doesn't have enough techniques on him to counter Itachi who can use crow clones to distract and cast his genjutsu on him as he is shown to be weak against it. Itachi can even use his finger to cast his genjutsu, and at point blacnk range, A is literally screwed as he's a close-range fighter with zero long-range techniques. By this time, Itachi is still without his Susanoo, hence he hasn't lost any significant chakra.
Onoki is a sluggish fighter, and would try to take out Itachi with Jinton. If he starts to prepare it or Lighten Gaara's sand as a counter for his Susanoo use, he is taken out instantly with a single Amaterasu. If he decides to distract Itachi with golems then Sharingan can see Chakra behind hinderences and also gives colour to Chakra. His distractions aren't working against a Sharingan user. Gaara's sand is weak to extreme Taijutsu feats, how it is moving fast enough to Shield another is beyond me, henceforth, don't even throw this logic at me.
The actual problem is Gaara here. For Him Itachi will have to go under Susano'o - if he wasn't already inside it after taking out A - and use Yasaka Magatama repeatedly to break his defenses. All of attacks are rendered useless before a CS with Yata Mirror. In fact even before going into CS mode and before taking out Onoki and A, Itachi can go into basic Susanoo form and remain mobile as we saw when Sasuke chased him through the trees, he and Sasuke had their basic forms active with a single hand.
This form can shield Itachi from tougher attacks. Regardless, Itachi can use Crow clones here as well and light Gaara's sand with Amaterasu, that will eat through his chakra that he is flowing through the sand to control it. I say it's fifty fifty, because it all matters how fast Amaterasu makes Gaara lose his Chakra and how fast Itachi succumbs to MS strain.
I find this Base funny. At one end Itachi is over-rated to such proportions that it's too funny, but at another end, he's underrated to impossible proportions. What Itachi has to his advantage: Base Speed, Speed Attacks, Crow Clones using minimal chakra and using all of Itachi's techniques - Finger Genjutsu, and Yata Mirror Equipped Susano'o. What The five Kages have going against them: Three extremely sluggish members, especially Mei and Onoki who have no counters to genjutsus. A's vulnerability to Genjutsu, and Gaara's sand and its weakness to extreme speed.
What team B has in its favour is basically Gaara and his infinite supply of sand, but it requires a monstrous amount of chakra, as we saw that when he used an large amount to protect the village, he nearly croaked. What Itachi has as his weakness is MS itself. This is how I believe this would go.