A curious design choice by the developers. Whenever a character uses a support skill that affects other allies that character will get 30 exp every time, regardless of current levels.
Therefore the easiest way to grind is to backtrack to an early encounter battle, kill every enemy except one and then spend the entire time using support skills, such as Heal or Attack Up, that catches at least one ally that is not the user in the effect. You'll level up every four moves used, and since MP is restored everytime you level up you can do this as much as you want.
The only characters not capable of doing this are those who do not possess support skills that affect others, such as Saki whose support skills only affect herself (and therefore only yield 5 exp).
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.