Gain Easy Experience.
While exploring dungeons you can create auto-run pathways. You use this to create a closed loop that makes your party run around until you decide to stop them. While your party are running in circles aimlessly they will encounter enemies. When holding the A button, you will rapidly select attack (melee). Get something to clamp down A, and you will be able to fight monsters while the game is in idle mode. When you return, you will have gained experience. The amount of experience is directly proportional to the amount of time you let your party run around, how much experience the monsters are worth, and the rate of encounters.
Note:Make sure your party is strong enough to survive each encounter, and have a healer present.