Tap Up, Down (x4), Right (x8), Left (x16) during the title screen. You will hear an explosive to confirm. Press the Start button to go to the level select.
You must have at least one bomb. When you come to a boss/enemy detonate the bomb and instantly pause the game, so the explosion wih the skull is on the screen. Leave it paused for about 15 seconds, and when you unpause, they will be destroyed.
When the Push Start Button text on the title screen is flashing, hold A + B + C and lightly tap the Start button. The difficulty selection text appears. Now, release A, release B, hold A, release C, hold B, release A, hold C, hold A, release B, release A, hold B, release C, hold A, release B.