Flayer Youngling
This is my favourite pet for leveling. In Veiled Stair (where only critters spawn) he can easily beat the three pets of the battle really fast with his "beast" type skills. I get around 900 exp with the difficulty bonus. Awesome!

Just go to the veiled stair with a lvl 25 flayer youngling or other pet with beast type attacks. There will only spawn critters so it is veeeery easy to kill them quickly. I get around 500 exp from each battle just switching the pet after the first turn, it´s too easy xD