Once the Water Dome comes up (And it definitely will as Kisame can easily survive until he decides to use it) its over for him.
-Limited Mobility in the water compared to Kisamehada, he can easily and effortlessly evade all of this guy's attacks.
-Water washes off poison, though any IS can be evaded with ease in the Dome.
-Kisamehada is a sensor, can absorb anything he comes in contact with and Sasori uses chakra strings to control his puppet. If Kisame so much as swims in between Sasori and his puppet, the chakra string will be absorbed and he can promptly destroy the puppet and end Sasori.
If all else fails, which it won't, then GSB is fired off in the Water Dome and its GG for sure, as Sasori's core chakra and puppets strings will be absorbed, so he dies.