In Theory could Kissame  fight pain and win?
 
Well Kisame could get physicall with any of the Paths and i pretty much see him defeating them all on a one on one basis besides up to Deva Realm who would be the real challenge, I can see kisame pretty much dealing with all of the summons by animal realm somehow by drowning them and attacking them with his shark jutsu only the amplification would be a prob but once kisame gets all close on that relam he's screwedi can see kisame attacking with tidal waves of water like when he was attacking team guy to keep the paths seperated for one and once he fuses with the samehada and catches all of the paths in his gigantic water prison none of them could escape and i can see him destroying them all besides DEVA REALM who wouldnt be touched by any attacks but i can just see kisame abosrbing all and any of his attacks so deva would break the water prison with a shira tensei but kisame in fused mode would absorb the attack and would just outlast nagato with stammina until all of the paths or dead  and then he would just hunt down nagato and then nagato would be screwed.
 
 
And lets say kisame dosent know about the six paths like jiraya i can see kisame getting sneak attacked and suddenly overwhelmed by the six paths like any normal foe because when they are fighting strategically like when they fought jiraya we all see what happens, but if they all on the battle front just attacking liek when pain fought naruto i could see them going down eventually after a very very very very very serious battle but  ultimately kisame winning because of his endurance.