At the main menu password screen, enter the following.

Row 1: Aang, Zuko, Toph
Row 2: Aang, Toph, Toph
Row 3: Zuko, Momo, Iroh

This opens and gives an automatic A-ranking on the first 12 levels, allowing the secret level, "Single Run" to be played. Single Run is not a new level but rather all 12 levels strung together. They are played one after the other without stop.