Legolas (no bow) vs Oberyn and the Mountain
Legolas with his twin blades (no bow) in a trial by combat scenario.
Vs
Oberyn
The Mountain (non-zombie)
There is no vendetta between the Viper and Mountain so they will have no qualms about fighting together.
Both sides are also given a month to train. That mean Oberyn and the Mountain will get familiar with each of their fighting styles.
Both sides are shown how the other fights, so they will each know what to expect.
Fight in the same arena where Oberyn got his migraine.