When your party receives the boat and can go on water to fight water-based enemies, you will be able to easily level-up. Find a place where you can travel around the entire world map via water without running into land. Tape down the directional button and the X button (or put something else heavy on it), making sure your party is around level 20 or higher in order to withstand the attacks that will occur. Then, walk away, keeping the occasional eye on the game to make sure your party hasn’t been slain. If done properly, your party will quickly and easily level up in this way.