Kudos for developing and maintaining such an awesome roguelike! After playing with a bunch of different combinations I realized the secret is obtaining the Open Dimensional Portal spell. Once you build up enough Intelligence to cast it a few times the game completely changes as you can ensure that you don't go too deep.
Using Portal you can stay in the 10-15 dungeon level range and build up your skills, stats and spells. Once you get stronger and find some anti-death protection you can cruise in the 20-25 dungeon level range which is the sweet spot for leveling and obtaining items without too much danger.
I think I could have literally played forever but I got bored spamming create food hundreds of times to get boar meat. Boar meat is the only way to level remotely quickly at very high levels due to it providing 20% of the needed experience for your next level.
You might want to think about either nerfing Open Dimensional Portal to some degree (more expensive to cast or lower the % chance) or making very powerful spells only castable once per dungeon level.
Even though my character started as a melee focused Axeman, I wound up pushing casting skills/Intelligence and using the heck out of these spells:
Clairvoyance
Conjure Spellbook
Gain Knowledge
Improve
Open Dimensional Portal
Teleport
With my roided out character, one Cleave Soul and one dual ax attack made quick work of X



