Get Cardinal, go to the final boss fight. When he's on his last form, let him kill you. The castle crasher will fall, and Cardinal will find the boomerang in a rock whilst you fly down. You'll have to complete the boss again from the takeoff to save it though.
Holy_Hobo kicks more ass than Captain Falcon on Steroids -T4UR3N http://img359.imageshack.us/img359/682/brawlsigcopyfu5.png