To fight ''Nightmare Geese'' you must get at least 3 SSS Rankings,2 AAA Rankings,get over 1,000,000 Points,and never get a ranking below A.
Enter the ranking screen at the high score table. Quickly press Left, Down/Left, Down, Down/Right, Right, Up/Right, Up, Up/Left, Left, then hold Up/Left and press A, B, C. Geese will say "Come on" to confirm correct code entry.
At the character select, move to the character you choose to play as, then do the following: Hold the start button (don't let go of it!) Tap buttons in sequence: B, B, C, C. Now hold down the B button for about a second. While still holding B (and Start), press and hold the C button. While holding these three buttons, press A or D to select your character in Shadow form. You may now let go of all the buttons. (A or D is used to select the color of your character's clothes)