I made it to level 20 yesterday and decided to give the wizards another shot. My first encounter was with three wizards and I managed to kill one of them before I died. That gave me hope, so I went in again and ran into the optimal two wizards. I managed to beat them down with plenty of health to spare! Victory! I took the crown, drank to full, and went to leave the dungeon. On the way out I ran into some monsters who can paralyze me. They went before I did, paralyzed me, and then slowly beat me to death while I couldn't take any actions. I'd won the battle but not the war, it would seem.
That was discouraging, but I went back in again. A couple tries later I ran into two wizards and they beat me. I tried again. Two wizards, and I won! And then I got paralyzed on the way out and died. I made it down to two wizards twice more, and they beat me both times. At this point I'd spent a couple hours trying with nothing at all to show for it, so I gave up. I played some League of Legends games, watched some Battlestar Gallactica, and then came back. I looked up the stats for the monsters that were paralyzing me and determined another level or two and they'd probably start running from me before they managed to beat me to death so I went to level up some more. This would also help solidify beating the two wizard fight since I was only 2 for 5 as it was.
I gained one level, and then took note of how fast I was gaining experience at level 21. It seemed like it would take an hour and a half to level which was depressing. (It turns out anything I have a chance at beating is scared to death of me and runs away, so not only is my tnl going up by huge amounts but the amount of experience I get per fight is nosediving.) So I went and gave the wizards another shot. This time I beat the two wizard fight and actually ran away successfully from the ghouls in the dungeon and made it to the surface. Victory! I have my crown! I'm one trivial boss fight away from getting the mystic key and with it a huge weapon and armor upgrade, along with access to more places to potentially level up.
Only it turns out that one trivial boss fight is absolutely not trivial for one thief. Normally I have to level up to beat the marsh cave, so when a normal party is good enough to get the crown it's more than good enough to kill Astos. But he's nigh invincible against a thief. I'm not sure it's even plausible to win, though I will look into the numbers further tomorrow to see if I even have a shot at level 28...