The Duel is a useful training ground for familiarising yourself with the various enemies and bosses in the game, as well as being a good challenge in its own right.
With two players it's a versus mode: you can use all the various attacks to fight each other.
The single player is more interesting. You start with 80 hit points, and you cannot heal or use any magic. There are no credits either, so if you lose you have to start over.
There are fifteen rounds: