As far as I'm concerned, Boolossus can go screw itself 15 times.
Why? The method of defeating it is super tedious. You split this thing into 15 smaller ones that you have to suck up. Every time your vacuum turns on, the Boos seem to fly away 50 feet away and you have to constantly chase them down. Maybe you can freeze them, due to the fight spawning ice elemental ghosts. The ice projectile you shoot isn't very good, though, especially when there's a few Boos left and getting these things to cooperate is terrible. When you wait too long they reform into this thing and you have to drag it with the weird ass vacuum physics again to separate it.