I beat the Dragon with a max level Druid. Probably just used Fire Form for the resistances.
There's two more modes, each new one unlocks with the defeat of the Guacamole Dragon. Beating it on Normal unlocks Expert, which limits where you can save, makes you lose gold on fleeing, adds a hunger and sleep bar, and the biggest change - you only have 15 days to beat the main quest. Do that and that limitation is lifted, but the others remain.
Beat the Guacamole on THAT difficulty, and you unlock Wizard Mode, which is normal mode but you have ingame cheats at your disposal.