Beat game in one hit on the Poppy boss
When you get to the start menu, press up, select, and B. You will see both a white cat and a black cat walk across the screen. When both cats are off the screen, press A and start the game. You will find yourself at the boss, hit him once and you beat the game.
submitted by freshprince01

Bonus Games
At the title screen, press Left + B + Select. When the screen changes to the High Score screen, a white cat will walk along the bottom of the screen. Return to the title screen and begin a new game. When you select a stage to play, you will automatically go to the bonus game.

Boss Select
At the title screen, hold Right and press A + B + Select. You'll be brought to a Ranking screen with a little black cat. From here you can fight any of the game's bosses.

Easy Points
Go to the boss of the third pinball land (the one on the far right.) When he splits into two, defeat only ONE of them. Right after that, press select and save game. Then go back into it, and do the same thing over again. Every time you get 50,000 points. But be careful; don't destroy the other boss or you will not be able to do it.

Erase High Scores
At the high score screen hold START and SELECT. If it takes you back to the title screen continue holding these buttons until the high score screen appears again. At this screen, while holding these buttons hold DOWN. If it worked a bomb will appear and an animation will commence.
submitted by The Code Wizard

Only Bosses
On the title screen, simultaneously press right, select, a and b. If entered correctly, a black cat will run across the high score screen. Press start to begin a new game. You will go to a screen with the bosses names on it. Hit kirby to a star to begin