Godzilla

To fight all final monsters, enter "DESTR0Y ALL M0NSTERS."
To meet the Monster Zero at the start of the game, enter "M0NSTER 0."
To view the opening and closing scenes, enter "START T0 END."
For the sound test, enter the password "S0UND"

Just enter the letters to your password and ignore any irregular spacing that occurs between the words.
Notes: Use the number 0, not the letter O. There are 4 lines to enter these passwords on. Always start entering the password on the first line, and leave any unused spaces blank.

At the password select screen, put in NO characters. Go directly to END. This will take you to the Earth level, with no monsters or any characters whatsoever visible. Press SELECT to bring up the password screen. Since you have no allies left, pressing SELECT to undo the password screen will automatically result in a GAME OVER.