At the 'Main' menu enter the following passwords at the 'Password' screen. The first 8 levels you'll recieve an automatic A-ranking which lets you play the next 5 levels.

Row 1:
Enter Aang, Momo, Momo.

Row 2:
Enter Katara, Sokka, Sokka.

Row 3:
Enter Iroh, Sokka, Iroh.