Jaime and Bronn vs. S4 Mountain

Post-Locke Jaime... but he carries Oathkeeper.

Bronn with a regular sword and a knife.

Mountain in the full armor, helmet included, and 2H greatsword. As we saw him against Oberyn.

Duo or Clegane?

Team.

They're too fast and skilled, and more importantly they wouldn't hesitate to kill him.

While Jaime keeps him distracted, Bronn hops on his back and jabs his knife into his eye holes a few times.

Bronn has a chance solo. Quick enough to hop around and kill Gregor like Viper, and he wont monologue or be distracted. Like he said though, one misstep and he's toast. The pair together for sure win, though Jaime likely gets killed as a distraction.