In Mario & Luigi - Paper Jam, there are numerous boss fights. If you want to unlock and defeat the secret Boss, then you have to invest a few hours in the game.
To fight against the secret boss, you have to play the game until the final battle. This is necessary so that all other bosses are added to the battle ring. In the arena you can compete again against some bosses - but the bosses have a higher level than in your first encounter.
Now it is time for you to defeat all of the bosses in the ring - the rank achieved here is not important. Once you have done that, the final choice will be unlocked: the Boss-Medley!
In Boss Medley, you have to fight against all the bosses again in sequentially. That makes things very tricky, because you have to divide your BP and items well. Ensure that you have the matching cards in your deck. A card that charges your BP, would be beneficial so that you can use in the long term the Bros and Trio attacks. The recommended level for the Boss Rush is 50 and you should train at least until level 46, so you can reach the last rank equip with two bonus extras.
Here, you should not have many problems with the right card.
But Dry Bowser does not come alone. After some time he conjures with Morton, Wendy, Ludwig and Larry. Again, there is a matching card in order to reduce the level of Koopalinge.