When you beat the game watch through the ending/credits and when the word "Fin" pops up hit A and you will be prompted to save. When you save there will now be a star to the right of the file. Reload the file you will start a new game with all your equipment/characters/levels and if you had any blue hearts for characters they will turn back to red. A bonus area will also be unlocked north of where you start off.
To level up easily, cast any healing or stat-boosting spell or ability on a character. If the affected character(s) are at a higher level than the person who cast the spell or ability, it will give them 30 experience points.