I mean, Kisame without Samehada took a direct beating from Guy in 7 gates, yet not only did he survive but he mustered enough strength to break free from a wood stock. So in other words Rasengan=virtually effect-less on Kisame. Now, when Kisame is fused with Samehada, he can regenerate, so Rasengan will be even more effectless:
You must be registered for see images
Plus when he's fused with Samehada Chakra based Ninjutsu won't work on him at all:
You must be registered for see images
FTG will be rendered useless if Kisame makes a Water Dome to drown the Kunias in. I'm being serious, kisame fused with Samehada has one of the highest chances of beating Minato. He may not win due to Minato's intelligence, but from feats displayed Kisame has an advantage over minato's displayed attacks.
In my opinion Minato is overall stronger, but Kisame is Mnato's ultimate worst match up. minato can beat kisame, but with the most difficulty he'll ever see in his life