There is only one way in

Answer a ringing phone

Find someone to play remote Batarang catch with

A moment of remembrance

Complete story mode on Normal difficulty

Complete story mode on Hard difficulty

Defeat the giant Titan Ivy plant

Venture into Killer Croc's lair and come out alive

Defeat two Titan Henchmen at once

Capture and lock up Harley Quinn

Complete the quest "The Neoteric Witch" and AAC Light-heavyweight Tier.

Complete the Tower at Paradigm's Breach.