There is only one way in

Save the damsel, but is she in distress?

Enable European competition in the first season of Career

Unlock a Skill Challenge

String up one henchman and drop him to surprise a second (any play mode).

Complete story mode on Normal difficulty

KO all the henchmen celebrating your arrival at the party

Defeat the giant Titan Ivy plant

Ride Titan henchman and knock down 10 thugs (any play mode)

Conquer the effects of the Scarecrow's fear gas

Defeat two Titan Henchmen at once

Capture and lock up Harley Quinn

Solve 25% of riddler challenges