Upon beginning the level, enter the smaller building and take the elevator down to the basement. Follow the hallways here until you reach the merry-go-round, although there are two entrances to it. Neither is more convenient, so pick the closest one. Whenever you enter the merry-go-round, you will see a bunch of the Boo pictures along the outer wall. Most of these are for show and do nothing, but one of the pictures will actually spew out flames from it, so be mindful of it. What you are looking for down here is the picture that has Boos coming out of it.
There will be a total of five Boos coming out of that picture, so you can kind of sit in the same spot and continually Ground Pound them, although the ground moving makes it a little weird. Once you have defeated the five normal Boos, a Big Boo will pop out from behind the picture. This is pretty much the same battle as during Go On a Ghost Hunt, as you need to damage the Big Boo three times. It’s not really a threat, as it can only move towards you and should you touch it normally while it’s solid, then you will take some damage. Once you hit the Big Boo three times, out will pop the Power Star.