When you arrive in the Inverted Castle, make sure you have the Shield Rod and the Alucard Shield. After acquiring these two items, go directly to the Clock Tower Rooms. Here you will find Guardians. They are a high level enemy but give out a large amount of experience points. Making sure your MP is full, equip the Rod and Shield, then press X + B activate the shield's power. It is now invulnerable. Walk into the Guardians with your shield. They will die instantly. Watch your magic meter. If you run out, hide in a safe area and repeat. If you are a level 28, you will reach level 61 within twenty minutes.