Just my discipline, I once created the same thread.
Though it would seem that Orochimaru is chanceless, after thinking it thorugh properly and considering all of Orochimaru's abilities, this battle would be really interesting.
If Orochimaru hadn't the ability to regenerate his entire body he'd be crushed with no doubt. But his monstrous rebirth technique allows him to totally renew his body, making any damage or injuries andone. It's also a great way to trick and kill bodies, for example if they jump on him like they did with Jiraiya. Apart from all that Orochimaru is more than just robust. He survived a deadly punch from Tsunade, a punch in his face from Kyuubi Naruto, and even after Kyuubi ripped him appart he could put himself back together.
That ability combined with his sword of Kusanagi, Edo Tensei, the huge sommonings including the great defense Sanju Rashomon and his thousands of small serpants gives him pretty much ways to stand his ground against Pain and take out several of his bodys. In my oppinion his chance of victory is 50/50