Instructions
Reach level six, "A Harry Situation." Once you reach the level, Harry (aka The Green Goblin) will appear and take you for a ride on the glider.
Press the displayed buttons to fight Harry. You must press them immediately after they are displayed in chronological order (Triangle, X, Triangle, X, Triangle). After this, you will fall to the street.
Avoid traffic with the L2 and R2 buttons. Once the traffic is gone, press the displayed buttons in the order they appear to get back on the glider and continue your fight with Harry.
Repeat the process (you'll fall from the glider again) to defeat Harry. Once you defeat Harry, he will be accessible from the game's main menu.