Instructions
Collect eight Power Stars as Yoshi, and enter Princess Peach's playroom located in the main foyer of the castle. Enter the door marked with eight stars and jump into the Mario painting to challenge the king of the Goombas, Goomboss. Use Yoshi's tongue attack to slurp up small Goombas and spit them at Goomboss. Hit Goomboss three times, and he will drop the Mario Key used to open Mario's locked door.
Enter the mansion found in the Big Boo's Haunt level as Mario on course two or higher. Climb the stairs and go into the room on the far right. Grab the Power Flower and fly up onto the ledge above. On the other side of the doorway is a Luigi painting. Jump into the Luigi painting and find your way through the hedge maze to challenge Big Boo to a boss battle. Face a mirror to make Big Boo turn solid so Mario can hit the ghost. Hit Big Boo three times and he will give Mario the Luigi Key.
Bring Luigi to the mirror room where the Snowman's Land painting is located. Jump to the top of the room's pillars to grab a Power Flower. Use the flower to walk through the mirror and enter the Wario painting on the other side. Take care not to fall off of the icy platforms on the way to fight Chief Chilly. Use Luigi's punching and kicking attacks to knock Chief Chilly off of the platform. Each time he is pushed off, the arena is made smaller. Push Chief Chilly off three times, and you are rewarded with the Wario Key.