Shiny RoboBowser

Shiny RoboBowser is an enhanced version of Bowser and the final boss of Mario & Luigi: Paper Jam. After Mario, Luigi, and Paper Mario fight both the real-world Bowser and Paper Bowser for some time, Paper Bowser fuses with many of his paper minions, which Bowser then joins with to form the boss. Shiny RoboBowser is a giant version of the "real" Bowser, wearing cardboard armor that resembles himself (apparently formed from Paper Bowser and his minions). During battle, some of the minions can break off and join the fight. While the cardboard armor still has its helmet, Paper Mario cannot use Trio Attacks, as Shiny RoboBowser will counterattack Paper Mario's hammer with his own and and trap Mario, Luigi, and Paper Mario in a similar setting to the Trio Attacks.

Trivia

 * Shiny RoboBowser is the third final boss in the Mario & Luigi series to be a fusion of Bowser with another character (or in this case, multiple characters).
 * Additionally, Shiny RoboBowser continues the series' trend of the final boss being a different form or derivative of Bowser, following Bowletta (Mario & Luigi: Superstar Saga), Shrowser (Mario & Luigi: Partners in Time), Dark Bowser (Mario & Luigi: Bowser's Inside Story), and Dreamy Bowser (Mario & Luigi: Dream Team).
 * Shiny RoboBowser is the only boss in the game that does not appear in a previous Mario game.