This is certainly an interesting match up. . . A few questions though, is the Second Mizukage willing to cooperate with Orochimaru? Does Nagato/Pein know of Edo Tensei and the only way to beat it?
If the Mizukage is willing to work with Orochimaru and Pein doesn't know how to defeat the Edo Tesnei'd kage right away. .
It could go either way. I can see the Mizukage's genjutsu countering the combined visual powers of Pein while Orochimaru makes a B-line for either the Naraka, Preta, or Human paths. But the Genjutus would NOT last forever, eventually I could see Pein's animal path summonomh the several large animals it has, and just outright stomping around for the clam. Once the clam's done, the Mizukage would probably pull out Joki boy. Which would be easily countered by the Preta path. And if either the Preta path or the Human path remains after the genjutsu is worn off, I find it hard to see Orochimaru and his edo lasting.
The Human path would seal/steal the soul from the Second Mizukage and if the Preta is around, he would absorb Joki Boy, which would be the biggest threat to Pein. With the only things we've seen from Orochimaru, his summons and his Kusanagi sword, I don't think he would fair too well with out the Mizukages support. I'd give this to the six paths of Pein with high difficulty atleast 6.5 out of ten times.