PAL - Front
PAL - Back
Just before the fourth boss does his attack animation, pause and go to the character select screen. A message stating that you can't change your character will pop up. Press A, and you'll still be in the pause screen, but the boss' attack animation will be unpaused and he will continue attacking you. You are invincible whilst pausing, so you can just wait until he finishes his attack. You need to do this every time he does a new attack animation. Repeat this until he dies.