The most powerful hero of the Dungeon faction.

It seems that the Elves don't really have any strong late-game heroes. Well, only Lethos, and Erin, but that's a stretch. The Lizardmen's Sorgala has a significant drawback: the "bite" ability doesn't really fit with the concept of Lizardmen as fast and weak warriors, created to destroy stronger units and die almost instantly afterward. Although, if you think about it, nothing really fits with the Witches either; they aren't really good fighters, only their spellbook is a plus.
For a short game, I would take Vaishan, and then switch to the first Chaos Knight that appears in the tavern, giving him the lucky charm. This scheme will also work for a medium-length game.
I don't know about duels; there's a guide on the site, but it's probably outdated. And I personally don't enjoy duels.