What is fastest way out and can I come back later (at a higher level) to collect all the goodies and more XP for killing remaining monsters?? DO I need to go down to the level with Lichs?? Or can I save that until I'm higher level?

I really don't know why game devs did this: trapped in a very long, very tough dungeon with no way to replenish supplies or sell loot. Don't like it!!!