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.
Entering "X Y S"(including spaces) on the password screen will result in you being sent to an unfinished board. Godzilla and Mothra will not be on the board, so use the select button to get a game over and return to the main menu. Once you do this, go to your LAST GAME, and Godzilla and Motra will be on the board... somewhere.
Most of the board tiles will be hidden in this state; using the select button may make things easier to see. Mothra can be found in the top right of the board, but don't move her just yet. instead, select her, and fight one of the nearing bosses; there are several of them in the dark.
...after engaging in combat with one of them, the board will once again brighten up a bit(once again, the select button may help too). Vy the time every level on the board has two colors(excluding black), it should be safe to move Mothra around. Get both monsters to a base; Godzilla can simply be selected to move onto the "next level", but Mothra makes things a bit more complicated.
Move Mothra to a base; this shouldn't be hard, considering she can move many spaces. NOTE: do not move Mothra to any bases that have black in their palate. This will result in a soft lock.
If you are successful, you will be able to move Mothra onto the next field, and for your troubles, you will receive... a game over. There was no level programmed to come after this one.
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.