Obtain the Fidelis Aranea Glyph from the Skull Spider, then head to the Monastery. To the right of the save room in the top left corner of the map is a room that will spawn Zombies infinitely. Stand on the ledge at the entrance into the room so that the Zombies can’t collide with you and summon the spider glyph. After that, plug your DS in and walk away, allowing the spider to do all of the work. When you return hours and hours later, you should have accumulated tons of experience.