Well, if Nihilus wasn't busy supporting his ship, controlling his slaves, and being triple teamed, I'd say he would destroy Windu. My reasoning? Nihilus was incredibly powerful, able to destroy all life on Katarr via the Force. Though Nihilus was destroyed by the exile, it is because he tried to feed from the exile, who is, in essence, a wound in the Force. That motion weakened him. Otherwise, I believe he could contend.