Unlock the Bosses
At the team select screen, set the cursor on Mezu. Move left, up, right, you'll be on Gozu. Circle the screen once counterclockwise, returning to Gozu. Press D, you'll hear Jyazu's laugh. Jyazu is above Gozu's panel, tap up to access him. (up again to return to Gozu) It's the same for Shin Shishioh, who is above Kage Shishioh's panel.

NOTE: If you do this, you will not be able to save the game, enter your name in the high score list, or see the ending.