Walkthrough: After crashing the wedding, Mario will drop into an arena for the final battle of the Main Story, the same where he is finally pitted against his archival. Bowser will waste no time after the battle begins, quickly tossing hit fighting hat right at you. Swiftly toss Cappy to neutralize the hat, allowing Mario to wear it for himself. From here, the battle will be much like your first with Bowser earlier in the storyline. So, with his hat, try and punch at Bowser. He'll quickly bounce around and create flaming shock waves, followed by throwing a giant brick sphere. All of this can be dodge with perfect timing, and then proceed to punching him. Bowser will have learn since your last fight, blocking your first few punches. Jump his ensuing tail spin, and then punch him backwards.
He will quickly get back on his feet , hoping to the middle of the arena and creating a powerful fire breath that he'll bring from side to side. This can be a little tricky to dodge, but if you side somersault from side to side as he does, he'll stop spinning and doing this attack soon enough. The second phase of the battle will then begin, he will summon more hats and toss them at you. Cappy can destroy the fake hats, or you can simply dodge them as you focus on hitting his real hat and gaining control of it. After you begin to chase him, he'll send a number of flaming spiked shells your way. They can't be destroyed, so simply dodge them as you work your way closer and closer to him. As you reach him, he'll block the punches again. This time, he'll do two consecutive tail spins. Jump them both, and then send him backwards with a few punches.
As the third and final phase of the battle begins, Bowser will have an even longer fire breathing attack. After avoiding it until it concludes, Bowser will toss even more shadow hats at you. Do as before and eventually stop hit real hat so you can control it. Once you begin to chase him, he'll throw continuous flaming spiked shells that you will need to dodge until you can get close to him. He'll block your punches and then do more tail spins; however, even after avoid his tail spins, Bowser will get back up to attack once more. Go for more attacks, sending the final blow to knock him out and reach Princess Peach.