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.