In Mario Party 9 for the Wii, you can unlock the Boss Rush mode, which puts your team of fighters against Bowser’s many bosses in an all out cage match to the death!!
To unlock Boss Rush mode, you need to beat all bosses in the game at least once, and then look in the Museum to unlock it for 500 points.
Check out my other guide for the complete list of unlockables in Mario Party 9 for the Wii.
