Instructions
Enter the Mirror Room for the first time through the Fortune Teller's Room off the main hallway. The doors will lock after you enter and you will be attacked by invisible ghosts. Use the mirrors to locate the ghosts and suck up all of them to reveal a treasure chest.
Open the treasure chest to obtain the Fire Element Medal for your Poltergeist 3000. Afterward you will be attacked by a King Boo. Flash the King Boo with your flashlight, then suck him up in your Poltergeist 3000.
Light all the candles in the room using your newly learned fire ability to open the door. Return to the Fortune Teller's Room and light each of the candles to reveal a hidden key. Suck up the key using your Poltergeist 3000 and exit to the main hall.
Light the floating candle in the main hallway to get the butler ghost to appear. He will run away immediately, so chase him. Open the locked door using the key from the Fortune Teller's Room and continue your pursuit.
Chase the butler ghost back to its room. The ghost will be weakened from the chase, so hit it with your flashlight and suck it up with the Poltergeist 3000 to finish it off.