Instructions
Fill your vacuum with the ice element before you fight the giant Boo. To do this, suck up Ice Boos. Find Ice Boos in cold places, such as inside refrigerators.
Hold down the "R" button to turn on the vacuum and hold the giant Boo in place. While the Boo is trapped, shove him into the horn of a unicorn statue. The giant Boo explodes, forming 15 tiny Boos.
Face a tiny Boo and hold down "L" to spray it with ice. Suck the frozen Boo into the vacuum. The tiny Boos combine and turn back into the giant Boo.
Push the giant Boo into the unicorn statue again. Keep freezing small Boos and sucking them up. When you suck up the last Boo, the boss is defeated, and you get the key to the next area.