Have you beaten the final bonus stage yet? It's pretty difficult to be sure... In order to get here, you have to collect all of the Cheat Sheets in Level 3 and then go to this stage from the Level Select Screen. If you beat this level and have an 'S' rating on all other levels, you get to see ShadowMaster's Secret Bonus Ending! Are you up to the challenge?
The following video shows how to get a pesky Cheat Sheet in Level 3, Stage 4. This should be the last really-hard-to-get Cheat Sheet. So now you should be able to get to and beat the last bonus stage