If Samehada can cure Kisame of poisons (I doubt) he has a shot. If not Sasori will beat him.
As was mentioned earlier, trapping Sasori in the giant water dome will end with the entire dome being poisoned due to osmosis. If Sasori doesn't use the water dome then he is facing an army of puppets all of which are armed with lethal poisons. Kisame's water jutsu also wouldn't be able to wash out the weapon's poison since Sasori saturates his weapons in poison.
I also don't see samehada being particularly useful against Sasori. He could cut the chakra threads, but considering part 1 Kankuro could re-establish chakra threads in an instant, I doubt Sasori would have a problem. Also, I doubt that samehada can absorb chakra that is INSIDE of something that isn't organic or chakra based. At least it has never shown the ability to do that. Meaning that Sasori's heart should be safe.