The fifth bout with Fury Bowser can occur once you have collected at least 50 Cat Shines, and it’s considered the final fight of the “main story”. As always, once you have the required number of Shines and Fury Bowser is out and about, go ahead and collect any of the Giga Bells in order to begin the boss fight. Note that it’s possible that prior to collecting 50 Shines, Fury Bowser will come out to a red sky, meaning that no matter how many Shines you get, he will not go away. So, because of how annoying he can be and how difficult it can be to dodge his fire breath, it’s a good idea to quickly get 50 Shines and do this battle.
The majority of this battle is mostly the same as the previous ones, as he doesn’t really have any new moves. However, Bowser will prolong some of his previous moves. For example, his fire breath will be swept across the screen three times in a row, when he only did it once in previous battles. Another thing he will also do is do the slam and shockwave attack three times in a row before staying in the ground. That seems to be the extent of his attacks in this battle, so you can either damage him with the blocks dropped from the sky and/or by doing the Ground Pound when he’s upside down.
It looks like you’ve finally defeated Fury Bowser, but something else happens to Bowser as he retains his giant form and draws upon the power of the three Giga Bells. When you regain control, run forward and jump on Plessie, than a chase will begin. You will just have to control Plessie here, while dodging any attacks from Bowser. One attack he will perform is his fire breath, but this doesn’t seem to even come close to hitting you the majority of the time. If you do happen to feel in danger, you can simply dive underwater by pressing the Y button.
The other attack that Bowser will do is send those giant blocks at you, but he will be basically sending them as torpedoes instead of down from the sky. Dodging these can be a bit last minute sometimes, but it is possible to jump over them, as well as submerge yourself like mentioned above. Additionally, you can sometimes simply move out of the way, but he always sends more than one, so it’s best to just dive underneath them at that point. This chase will go on until you see a big ramp in front of you, where you will jump off the end of that ramp to ram yourself into the barrier protecting the Giga Bells.
You will have to do this a total of four times before the fight is over, although things will get progressively a little more difficult. Bowser will do his fire breath more often, while also sending out a lot more blocks as torpedoes. Upon hitting the Giga Bells for the fourth time, though, you will finally win over Bowser and complete the “main story.”
After finishing off Fury Bowser at 50 Shines, you will witness the ending and go through some credits. Once those are over, you will be back in Lake Lapcat, where Bowser Jr will mark any missing Shines you haven’t gotten on your map. Additionally, you will be able to get the rest of the Shines on Fur Step Island and you will also be able to fast travel to any of the named islands by simply placing the cursor over it and pressing the A button.