When I took on the dungeon to defeat Krokhotep by myself, I made it all the way through, to Krokhotep, defeated his minion, but lost to him.
I have a friend who says that when you are defeated in a dungeon, the dungeon resets and you have to restart the entire dungeon.
I found, though, that when I healed and restocked mana and potions quickly, when I went back in, I could go all the way to Krokhotep. He was without his minion, and was partially injured, the way I left him. I could then defeat him easily.
My friend says that is a glitch, that the dungeon should have reset after my defeat and did not, that the 30 minute rule only applies if you leave via teleport.
Dungeons must be completed to get full credit. Logging out, leaving through the Dungeon entrance, going to another dungeon, playing a Mini-game, going to a PvP or Derby match, or going to your home will reset the Dungeon. Leaving any other way gives you 30 minutes to return before the Dungeon resets.
If you want the prize, you have to build the ladder rung by rung. There are no shortcuts.