You're not factoring Roshi. He's a great asset against both Kisame and Deidara. Kisame's Samehada and water attacks can be countered by lava, which can be spit in huge quantities and in formidable size
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. It also affects Deidara because he can be machined from ground if he aims for the sky. As for Kisame, he's in a pinch because Samehada can't absorb lava, which it deems too hot, and he's forced to fight both Roshi in CQC with his lava armor, and B who can blow his chest off with a lariat. Eventually, he's either forced to create a dome or he dies, but Bijuu Hachimaki destroys the dome or Goku's lava evaporates it.
Deidara is not a factor in this fight, and he's a burden for Kisame. If he even tries to hurt B and Roshi, when Curse Mark tanked all his attacks bar C3 (but that kills Kisame too and Bijuu Mode can probably tank it), V2 transformations shrug off all his attacks. And eventually he runs out of clay. They have no way of hurting Jinchuuriki while Jinchuuriki do.