I would recommend the Zombie Mushroom Mini Dungeon, like Mitchi said. It's easy to get the Combo King Medal at an early level there (got mines at 15).
Crouching down will take a long time, and there's that chance where you accidentally kill your mini mob and they don't comb back; you'll lose your combo. Someone can also come in and attack your monsters (I accidentally did that once ._.)
I don't know how to make a cool looking signature D: