Ok ill tell you how to beat BOWSER.
First of all you have to get through his castle once youve got there you have to run under BOWSER when he jumps up in the air and push the button bowser falls in the lava above you is a cage with a whitch dressed up as peach it will break out and cast a spell which makes BOWSER HUGE.
You have to avoid his flames and punches the best way to avoid him is with the propeller suit or mini mushroom when you see peach at the end of the castles trail of levitating platforms jump on the pink button and BOWSER will fall deeper in to the lava and you have defeated him Peach will fall out of the cage and thank Mario/Luigi or Toad.
Later on Luigi and the two toads come out the castle with hot air ballons luigi will land and offer mario and peach the balloon when mario and peach enter the balloon it flys of without luigi and then the toads will land and luigi will enter there balloon. later peach says to mario "mario i hav'nt told you about the secret world", after that you will see all the koopalings roling BOWSER of his back on to his feet then the castle falls on him.
In the credits you get to play as Mario and you have to get as much coins as you can (i got 87)
After that you have to get every big coin from every level in every world to unlock levels for WORLD 9.
(ALSO YO CAN SAVE IT ANY TIME YOU WANT IF YOU GO TO THE MENU)
I hope this helped.