Got some time to play it this weekend. Got to the first optional dungeon, level 9. Mobs there level 16, auto death if they got a turn off. So spent 4ish hours there in the first two floors just hitting them over and over (dodging back when their stagger bar went down instead of attacking) until they were oneshottable with knight+warrior AE strike. Exiting the front door respawns the entire dungeon.
When they were out of MP, ran back and forwards in the first two floors just killing the enemies that died from just using the mage attack. At level 15 I started going through the entire dungeon as the loop (quicker as entering the place from the cat at the end also respawns the dungeon) so get a full dungeon both up and down, and at level 16-17 I no longer had to widdle down the enemies, I could oneshot them when attacking.
At level 19 I did the boss. I assume you can do this for every dungeon, so shouldn't ever be a problem to do everything you can do in a dungeon the first time you enter one. Left with 30-40k gold and mag (would have had more mag, but got all classes for all characters, figured why not). I like that you don't need to swap classes as once you hit master, you get a 1k exp potion to be used for any class for every 1k exp you get on it, so no loss.