Go and fight the machines in the library (second castle) until you get the MOJO MAIL (it helps to have the ring of arcona which you get from the room where you fight Beelzebub)

Then get poison gas (not necessarily in that order) Equip the Mojo Mail to increase majic attack power and go to the rooms to the right and left of the clock in the second castle.

Turn into poison gas and float over to the Guardians. To start with they give you 1500 exp and it gradually goes down.

When I first did this I was leveling with one pass through the room. Just be sure to watch your MP bar.