Why didn't Orochimaru fight Madara?
There are two reasons:
1)Orochimaru was busy reanimating the four dead hokages to bring strong reinforcements.
2)Even if he was as strong as the Kages or even stronger, but we see in the fight between Madara and the Kages just how strong Madara was. He singlehandedly annhilated all the five Kages. So if Orochimaru decided to face him alone then I am sure he would have died instantly.