To easily gain experience, go to the Desert area and equip the Girrafey animal orb. It will help you level up faster. This area has large amounts of scorpions. Continue to kill the scorpions repeatedly to level up faster. Note: You will level up based on how much damage you inflict on a target, and not the amount of enemies you kill.
Go to the point in the Lava World area where you need a sandwich to continue, but do not have a sandwich with you. Since you do not have a sandwich, enemies here will drop sandwiches when they are defeated. Do not pick up a sandwich and they will continue to respaw. Keep defeating them to gain as much experience as desired.