1. In my opinion its on level 6, the fire maze.
When you leave it, close and open the door again, most of the time two Ugguards respawn, which give 500 exp each (there is a crab too, which gives 450 optionally). If you have a frostmage, they are easily killed. You can normally spawn a third Ugguard, by clicking the first three buttons in the maze.
Frost skilled high enough, will freeze them very long, so that you can easily kill them. Don't forget, that every character has to hit them at least one time, for full exp.
You can leave level 6 to the lower levels from time to time, cause they endlessly respawn.
And in the near there is the snail-room for food.
2. The ice lizards on level 9 (650 Exp each)
3. The prison without releasing the endboss.
not sure: 4. Goromogs, these wizards on level 10: When you jump down a pit in the great room with the ogres in it and return through the stairs, there will spawn Goromogs (1000 each). This can be repeated as it seems. Having a frost mage in your party will again help a lot.
Would be a good idea, to take the orb from level 9, i think, which gives you 5 willpower. Combine it with the Conjurers hat, that should give you 2 Willpower, and a 2nd orb or the shaman staff(+2), and you will earn mass energy every time you level up.
It's sad a bit, that there are very few items with vitality on them, so that you can't really push your hp this way.