In the team edit screen hold Start and press Up+Y, Right+A, Left+X, Down+B .
Another Boss Code:
1. Finish the game (team mode)
2. Go to the character selection screen
3. Select yes on team edit and hold down the L&R buttons
4. Press up, down, up
5. The boss's should now be available
Random Fighter Choice:
While pressing the Left + Right buttons answer No when it asks if you want to edit your team.