Surtur vs. Mephisto

Who would win?

Inside Mephisto's realm, Mephisto wins 10/10.

In neutral territory, Surtur wins majority - 7/10.

Does Surtur have the Twilight Sword?

And where does the battle take place?

In Mephs realm he wins, outside he loses.

What the others said...Surtur takes this if it's outside of Mephisto's realm

Mephisto has feats to say he could win in his realm but I wouldn't give it to him over Surtur in anything resembling an average anywhere.

In his realm without Twilight would probably make for the best fight though.