3 Passwords for the Japanese version:
On the Japanese version, change difficulty to hard on the customise screen, then enter the following passwords (the final password seems to be gone from the English language version)

 

Password - Effect:
Longlongago. - Play against the last boss from part 1
!!!!DEMO!!!! - Play the demo mode
TEAMACT.SHOW - Shows a short demo introducing the team

Activate Death Helm:
Enter the password MFCL SYMC MSXF.

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.

Go to the last stage of the game:
Enter the following password MFMJ TVSY FVPX.

Play The Demo:
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)