> Pause the game with the Start button, then simultaneously hold X+Y to get the clean pause  -- hold X+Y again to return.

> At the main menu screen, select Gallery and on the first page, press B, X, Y, B, B, Y, X, B.  You should hear a confirmation tone if done correctly.  Now, go into Arcade Mode and you should be able to select the boss Stranger Plane.
(courtesy: GameFan)