As soon as you get through the Fortress of Kwamba, go to forest village and fight 6 Kobolds. This should take you up to level 25.

As soon as the game starts and you get to your house, leave and get some money from the cup and balls mini-game (30,000 to 100,0000 Potch). Buy the best armor available and get your weapon sharpened. Save the game then set out for some random battles. Try to kill three Bon Bons by yourself until reaching level 10. When you are close to level 10, go to Sarady in the Tigermouth. It is the mountain in the west. Go to one of the houses and find the man that gives you a Fortune Rune. Equip it to Cleo. When you do the "Fortress of Kwamba" leveling trick, she will go to level 33-40. If you do the same trick with only one other character (such as Luc) later in the game, he will go to level 50.