Much like Madame Broode, you will actually face Mario's greatest foe twice during your adventure. The first time will act as a warm-up, as both battles will be practically the same. In the first phase of each battle, Bowser will toss his hat at Mario in order to attack. You must counter this by tossing Cappy at the hat, and then adorning Bowser's hat for yourself. The second phase will see Bowser throwing numerous hats at Mario, forcing you to find the real one and doing the same as in the first phase (wearing it for yourself). The third phase will be just like the second phase, the only exception will be even more shadow hats that Bowser will launch at you.
After putting on Bowser's hat in each phase of each battle, you must run toward Bowser and use his own hat to attack him. The first match will see Bowser not being able to counter this, except for the final phase in which he'll use a shield to prevent damage and then attack with a cheap tail spin. In the second and final battle with Bowser, he will have his shield to prevent damage during all three times you will attack with his very own hat, capping each off with an additional tail spin. Each battle will end by landing a blow to Bowser three different times during each match.