To access to Musashi, the sub-boss in the game, go to the player select screen and press 'C' 6 times, 'B' 3 times, then 'C' 4 times within 3 seconds. If you did this correctly you will see 2 arrows at the sides of the screen and Musashi will be available.
At the main menu hold A+B+C+D then press START.
