Just last night I was at a loss for things to do in the game so I went to the Stillshrine and started a chain of the zombie baddies there (only got to like 140). Afterwards I was very jumpy while walking through the hallway in my house!
My friend says he got a 999 chain of bats in one of those control rooms in the secret area of the Henne mines (and leveled up a lot in the process).