Well. I’m not sure if I shouldn’t just give up. I spent TWO HOURS playing the end boss, and managed to get past his first phase only twice in that time. It is too random as to whether you’ll survive a rebound off of him, so you can’t rely on skill or practice to beat him – you just have to keep trying, over and over, and hoping that you won’t bounce off Bowser and down the plughole.
Accidently, I managed to pick up another Star and open the door to the left of Bowser. Didn’t spend any time in there though.