Healing after each match of coruse.
I think Itachi could defeat them all w/o KA
Itachi gave KA to Naruto so that's not really in his arsenal just like how Hashirama took Kurama from Madara so no need to say without, you should only mention it if you're using Itachi before he gave Naruto it.
If it's 1 on then I doubt he can beat all. All but Tsunade stands a good chance, Oonoki and A knows how to fight a Sharingan user the most. Oonoki can use wind, fire, earth or dust element so if Itachi is in Susanoo he can use earth jutsu to attack from below itachi since the yata mirror will block the dust jutsus. Oonoki flys so would be out of Itachi's strike range for Amaterasu or his blade to hit. A is fast enough to evade Amaterasu and it took Madara to get him in a genjutsu which he was surprised in so I'm guessing he wasn't barely to ever caught in a genjutsu. A and his team knew of Susanoo so they must have fought Itachi before.
Mei uses water, earth, fire, boiling and lava element. He Acid mist can pass in Susanoo as it did with Sasukes, it was melting Sasukes but he was in ribs stage so it may or may not pass full Susanoo but Mei can use earth jutsus to sink or attack from below Susanoo and use earth to block Amaterasu. Gaara already shown he can block amaterasu and can attack from below Susanoo. Gaara can use Sand coffin on Itachi from within Susanoo, only reason h pulled Madara out was because he's a edo and they need to hit him then seal him.
Tsunade has the worst chance because of Yata mirror. Also you have to note Itachi Disease made the side effects from his MS jutsus even worst which leaves openings and his max MS jutsus was 3 so he's dead if pushed too far like in his fight with Sasuke. Was already stated that his disease was eaten away at him and he used medicine to prolong his life.