Astral Hall 2F

If you haven't already, collect Mario's dropped items. Mario's hat can be found in a laundry machine in the Laundry Room (bottom left of 1F). Mario's letter is in a nesting box in the top-most area of the first floor.

From the Tea Room, take the 1F/2F stairs near it. Go left through the corridor, continue down, and take the first door on your right (Luigi's left).

It's recommended to bring fire, which can be obtained from the candle in the 2F corridor where many mice are present.

Check drawers and the chandelier for money.

Light the five candle-pedestals arranged in a pentagram formation.

Stand in the middle of the pentagram to trigger a cut-scene.

You will be attacked by ghosts: first an Orange Ghost and a Shyguy Ghost. Vacuum the Orange Ghost first, then deal with the Shyguy Ghost's spear.

After defeating the first two, two more ghosts will appear.

Finally, a White Grabber and two Shyguy Ghosts will emerge. It is recommended to defeat the White Grabber first.

Next, find and defeat the Boo, Booswoggle (100 HP), who may be hiding in a candle pedestal.

After defeating all ghosts, proceed right into a mysterious room.