Once you have collected a total of 20 Cat Shines, you will be able to fight Fury Bowser for a third time. As usual, just grab any of the Giga Bells in order to start the fight. Fury Bowser will have all of the same moves from the previous fight, but he will add one more to the mix. He will summon the blocks that fall from the sky, which become embedded in the ground. While you normally would use these to hide from his fire breath, you don’t need to do that as Giga Cat Mario. However, being giant-sized, you can actually pick up these blocks.
With one of the giant blocks in your hand, you can actually hit Bowser with it. Doing this will not only cause damage to him, but it will also flip him over onto his back, meaning you can perform a follow-up Ground Pound to deal even more damage. Since this battle is a continuation of the second fight, Bowser should have half health, at most, depending on how many Cat Shines you grabbed while Bowser was awake. Once you finally defeat Bowser here, you will unlock the rest of the map, although some minor spots will still be covered with the black goop by some of the islands.