There is only one way in

Have 12 murderous dates with Calendar Man

Become the Bat

Use melee or ranged weapons to take both arms off a living enemy

Find the Outer Way boarding craft

Max out a weapon's proficiency.

Get praised by Jessie at the end of the motorcycle mini-game.

Rise to the top of the Seventh Heaven darts leaderboard.

Collect 3 music discs.

Free a bound ally.

Exploit an enemy's weakness.

Stagger an enemy.