I changed the style a little bit and it works faster now:
I only kill the three Ugguardians (one of them set free by pushing the four buttons) in the fire maze, and the cave crab which spawns there (about 2000 exp, my fighter is level 17 now, the others 16).
Then i sleep for a little bit more then one minute. After that they are respawned. I leave the skeletons in their halls and only visit the snails for food (all level 6).