In his room you will find him in the crib, after you make him come out, hit him with the ball that is located on the floor by the door. Once you do this and suck his health he should bring you into a level where you are inside his crib. This is a plain arena, so there is nothing to hide behind, first he will send some rocking horses after you, they are really easy to dodge, then three balls will come rolling into the crib, most of the time only one ball stays in. Grab this stray ball with your vacuum by hitting the R button when you point your vacuum at it.
Once you have it aim the ball at the baby and release R, this should shoot the ball out and knock him unconscious for a second or two, when he is unconscious vacuum up his heart with the R button, at this point it is like a regular ghost, when he breaks free he will jump around the crib in a clockwise formation, all you have to do to dodge this is to follow him, after this it goes back to the start and horses come out again.
On the balcony, on the third floor, there will be 15 or so boos’ going around in a circle. There will be a cut scene and they will turn into a big boo, after the short video the door will disappear and you start fighting this boss. There will be two unicorns on this floor, one on the right and one on the left, stay close to one of these and you will less likely to get hurt, they spurt out an ice elemental, which you will need so fill up on it, the big boo will start to follow you around by jumping.
The way this works is the Boo is basically attached to a string behind you. Imagine it as having a gigantic Yo-yo bouncing towards you.. Circle around one of the unicorns until he hits the horn on top. Once he has hit the horn they boos will come out, here is where you need the ice element, you need to fire the ice at the individual Boos and then vacuum them up with R. Keep doing this until all of the boos are gone.
In the basement you will find King Boo in front of a picture of Mario. Go up to him and he should transport you to the roof. The roof has three buildings on the far side and pillars all around, you can use the pillars as shields for some of his attacks. Bowsers attacks include: fire breath, which is shot from his mouth in a straight line, Air take in, in this move Bowser takes a deep inhale and tries to suck you into him stomach, tail swipe, this is just there so that you can’t grab his tail like in Mario 64, spike bomb, in this move Bowser has big spike bombs that he throws at you and essentially they blow up on contact or after a period of time on the ground, ground pound, in this move Bowser jumps and comes crashing down on you no matter where you are, in order to avoid this just keep running in a straight line.
To beat Bowser you have to snatch one of his spike bombs with your vacuum cleaner using R . You will have to touch it so make sure that you are lined up properly or the bomb will go off, also it will blow up if you hold on to it for to long. Once you have one of these bombs hold on to it until Bowser lowers his head. It doesn’t matter which attack he’s doing. You have to fire the bomb at Bowsers head. Once he has been hit his head will pop off and King Boo will pop out. If it doesn’t pop off try hitting him a second and third time, then it is like a regular boo just vacuum him up, you have to be weary though because Bowser’s head doesn’t just lay there; it will get up and breathe fire on you while you are sucking health from King Boo.
Repeat this until King Boo’s health is gone and then you win.