The Boolossus starts out as fifteen Boos standing in a circle on the Balcony. After you go up to the Boos and push A, you'll be taken into the boss fight. The Boos will all merge into a giant Boo. This fight is longer, but once you master the pattern, it'll be easier to beat. First, you'll need to suck the giant Boo with your vacuum cleaner and lead him over to one of two ice unicorn statues on the balcony. He'll be trying to squish you, so keep running away from him and try to suck him after you towards the statues. Once he bounces into the statue, he'll explode into the fifteen tiny Boos again. Once this happens, you'll need to suck ice elements out of one of the unicorn statues. Shoot the ice at the tiny ghosts to freeze them, then suck them into the vacuum. Keep doing this until they form into a giant Boo again. He won't be quite as big this time. Lead him into the statue again so he pops. Repeat the freezing procedure to suck up more tiny Boos. You will keep repeating this for the rest of the fight. This fight takes some time to beat, and the tiny Boos get faster the more of them you suck up, but on the plus side the giant Boo will get smaller when he reforms. If you have a hard time freezing the Boos, suck them closer to you and then spray them with ice. Always stand close the one of the unicorn statues so you can get ice easily.