Believe in Bowser
From the Super Mario Wiki
Believe in Bowser is the tenth level of Bowser's Castle in the Minion Quest: The Search for Bowser mode in Mario & Luigi: Superstar Saga + Bowser's Minions, and by extension the final compulsory level of the game. This level is available after completing the main game.
Despite a seeming win against Fawful earlier, a Lakitu overhears that "Bowser" is actually Bowletta, and the Goomba Squad encounters Fawful again. He engages a trapdoor to send out his own squad of Mecha-Fawfuls, Lakipeas, Gritty Goombas, Beanerang Bros, Paratroopeas and Clumphs to stop Captain Goomba once and for all in the underground Mechawful factory. With the large troops approaching, Captain Goomba manages to rally his entire huge squad to fight one last battle to rescue Bowser. He succeeds, and the Goomba Squad advances to the real Bowser as Bowser's Castle explodes from the bomb that Prince Peasley set earlier, leading to the ending.