Repeatedly fighting the Bandit Boss or The Beast nets 13k+ and 9k+ experience respectively, tho they grow bigger with every fight. The Beast's size resets if you lose.
You can also go to the prop dungeon that's to the right of where you find the Blacksmith's Key and the first robot arm bandit. That place has most of the enemy types and is a good place to find Essence items. It's also easy to navigate even if you're giant.
Finally, I discovered last night a place across gorge bridge that has Rock Monsters that can give 13k+ XP for the weaker varieties and 40k+ XP for the stronger ones.
If you want to cheese early fights, I recommend employing one the 4 slot mercenaries from the tavern. Solon in particular has good stats that total 1250 levels worth. She's good by default, but if you raise her relation high enough, you can reallocate her stats and she'll bulldoze thru most fights for you.