To easily gain experience, go to the Desert area immediately after getting off the boat and equip the Girrafey animal orb. It will help you level up faster. This area is swarming with an endless supply of scorpions. Keep killing those scorpions repeatedly to level up faster. Note - You will level up based on how much damage you deal to a target, and not how many enemies you kill. This will require some time and patience.
To easily gain experience, go to the point in the Lava World area where you need a sandwich to continue, but do not have a sandwich with you. Because you do not have a sandwich, enemies here will drop sandwiches when they are defeated. Do not pick up a sandwich and they will keep respawning. Keep defeating them to gain as much experience as desired. Note: this trick works best with low level characters.