Culex, a hidden boss
To fight Culex, buy the fireworks for 500 coins at Moleville, then trade it for a "Shiny Stone" at Moleville from a mole somewhere near some crates OUTSIDE of the homes. Next go to Monstro Town and go to the third or "3" door and open it with the Shiny Stone. The shiny stone is required to open the door. Next, get ready for the ultimate fight! He is strong so I recommend training before fighting him.
Build Levels Easily
First, go to an area with a treasure box containing a star. Make sure you save somewhere close to the area. Now, get the star and hit as many enemies as you can. Then, die in battle and you return to where you saved with all your EXP! You can repeat the trick until you get your levels to where you want them.
Rare Item Shop
After you rescue Dyna & Mite, the man from the mine opens an item shop. At different points in the game, he finds new items. He can sell you a Lucky Jewel, Mystery Egg, and Frying Pan. The Frying Pan may sound useless, but it's actually Toadstool's best weapon.
A list of all of Mario's Allies + Strategy for a Battle Team
To be sure Mario has all of his allies, here's a list of them.
Now, I first recommend a team for battles.
Mario can be your power attacker, Geno can be your magic attacker and Peach can be your healer. Mario's job can be to attack enemies and use some items if necessary. Geno can use his special moves like Geno Boost to raise your stats. He can also use magic attacks on enemies. Peach can mainly heal and use items.
Now, you are wondering about Bowser and Mallow. Well for Mallow, he can mainly do Magic Attacks. Bowser can just attack, having a lot of power.