How do you beat the last level on Mario oddesy?

Here's how you beat the re-incarnated Bowser being controlled by King Boo. When he rolls the spiky ball at you, you have to suck it up with the vacuum (you have to touch the ball with the vacuum while sucking). When you have the ball, shoot it at Bowser. His head should fly off, with King Boo flying out of the body. Keep trying to suck King Boo up with the vacuum. He will eventually go back into Bowser. Repeat the steps until you win.

Good Luck!