Set the difficulty to Hard, then enter the password: B J Q X Y R K C D L S Z. This allows you to control the character and fight through the demonstration scene. You will have two chances to fight two creatures: a warrior on a horse, followed by a large skeleton dragon.
See The End Credits
Set the difficulty level to Hard, then enter the password: M T k M S k T k H N S H. (To get to the lowercase letters, select Page on the Password screen)
Fight New Bosses
Go to the Password screen and enter the following password: X x x x Y y y y Z z z z. (choose Page to select the lowercase letters) This allows you to fight Tanzra's final form, a giant Centaur. Then continue right and you will fight against the end boss from the original ActRaiser game.