Pain. Full power Jiraiya was able to take down three bodies, i consider 2nd Mizu little stronger than Jiraiya, but that wouldn't be enough to take Pain 5 bodies down.
How does Pain deal with a "full power" Mizukage
The animal path could just summon the panda to protect the paths from the explosion and the paths don't need to be quicker than the mizukage they can just stay in their formation and if they mizukage doesn't use any ninjutsu then the asura path could just fire some rockets at him and i doubt the mizukage's hydrifacation technique could be used to protect him from that.I'll give this to 2nd Mizukage high difficulty.
Joki boy was capable of out speeding Gaara's sand easily. His only way of slowing it down was with the sand.
I don't exactly see how Preta can absorb explosions.
I suppose the summonings are a good counter. But I haven't seen any speed feats from the summonings. But I can see the Chameleon being dangerous since it can go undetectable.
The animal path could just summon the panda to protect the paths from the explosion .