Use the following strategy to balance out your characters' strengths. When a character reaches an even numbered level (2, 4, 6), choose the HP bonus. When a character reaches an odd numbered level (3, 7, 9), choose the offense/defense bonus. When a character reaches a level that is a multiple of 5 (5, 10, 15), choose the magic bonus. This will balance your character's strengths very well.
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Make sure you have the Exp. Booster (buy from Frogfucious' student in Seaside Town) and Lucky Jewel (buy for 100 coins from the explorer in Moleville.) Equip your team as follows:
Mario: Lazy Shell, Fuzzy Shirt (or higher), Exp. Booster
Geno: Hand Cannon (or higher), Fuzzy Cape (or higher), Safety Ring
Toadstool: War Fan (or higher), Lazy Shell, Jinx Belt
Enter the fight with Culex. Use the Lucky Jewel sometime during the fight, and defeat him and the four crystals. When you win, you should have 244 experience points, and no coins. Then, play the Yoshi Bonus Game. Guess correctly to earn 488 experience points.
Note: This trick requires a controller with a auto-fire feature. After you defeat Croco and get Mallow's grandpa's coin back, go to Mushroom Kingdom. You will find a group of Shy Guys on pogo sticks (their names are Shyster). Make sure you are about level 6, and are equipped with the Hammer (Mario), Shirt (Mario), Jump Shoes (Mario), Pants (Mallow), and Antidote Pin (Mallow). Stand in front of the door of the castle, You will see a group of Shysters coming out of it. Set the auto-fire to A, so that when the Shysters collide with you , the battles will be fought automatically. Keep the system on to automatically get levels. Keep in mind that the maximum level is 30. You do not really have to worry about getting a game over while the game runs unattended -- when you level up, you will fully recover automatically. However, when you level up, you will get extra HP instead of choosing what you want, but every stat goes up anyway.