Instructions
Run to the right as soon as the battle starts. Hold down the "B" button to put together an anvil.
Face the grapple point that appears, and press "Z." Batman grapples to an overhead platform. Switch to Robin, and hold "B" to build a metal bar, then climb it to reach the overhead platform.
Switch back to Batman, and step on the orange button. When Robin steps on the button with you, a hammer smashes into Harley's bell, and she falls to the ground below.
Run to the right side of the platform, and hold "B" to fill in the gaps in the floor. Get behind the crate, and push it across the platform as far as possible.
Hold "B" to put together two metal bars. Climb them to reach a second orange button. Stand on it to knock the Joker out of the bell.
Jump down to the main platform. Hold "B" to assemble a water gun. Press "C" to use the water gun. Aim at the Joker's helicopter, and hold "B" to blast it with water. Shoot the helicopter with water until it crashes.
Run up to the Joker, and tap "B" repeatedly to attack him. After a few attacks, he stuns you. Switch to Robin, and attack the Joker while Batman recovers. Each time you hit the Joker, one of his hearts disappears. When all the hearts disappear, you defeat the Joker.