Itachi can solo on his own
just shut up you should have been banned ages ago for riding itachi too hard.
OT: Epic post , I would love to see this fight and I believe Itachi / Kisame Win High Difficulty.
Kisame infused with Samehada is extremely powerful , his power rivals that of a tailed beast which is what it took to defeat pain.
Itachi's Susanoo could very well defeat the animal path and summons. Itachi's genjutsu could also render other paths unavailable like Jiraiyas genjutsu did. (i am implying itachis genjutsu prowess are on the same level as Jman's sage genjutsu)
I believe that with their clones and powers they could defeat Pain if they really wanted to tbh. Against planetary devastation I'm confident that Susanoo would be able to break or if Itachi was being levitated he could use his visual amaterasu to ignite the caster(s) .
Kisame and Itachi compliment each others fighting styles. Also Kisame could envelop paths underwater similar to what he did to Killer Bee and have a HUGE advantage that way as well..
Overall : Itachi / Kisame HIgh- Very High diff