Select one player mode, choose the Vs. option and the hard difficulty level. Hold L + Up and press A or Start. The screen will turn red to confirm correct code entry.
Enter S6DJNB72 as a password to play on the hardest difficulty.
Hold L + R on controllers one and two for approximately five seconds at the player vs. player character selection screen. The question marks at the bottom of the player vs. player character selection screen will be replaced by images of the bosses.