Sega Master System / Mark III / Game Gear
During the title screen, press . A sound will confirm the code. Now, is possible to select the Extra Level (more difficult than Hard Level). Selecting this option, is possible use the boss The Alien.
Immediately after the Extra Level code, press . A sound will confirm the code. Now, is possible to select the Zero Level (easier than Easy Level). You can use the boss The Alien in this difficulty level too.
Alternatively, you can unlock this by getting a high score and entering the initials
See the main page for Buster Fight (バスターファイト)