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10 G

The best way to get this would be to play the new FREE DLC challenge "Totally Insane" - you fight against an infinite amount of enemies - hence the infinity loop icon on the thumbnail.

New enemies will drop in constantly, allowing you to chain together some very long combos.

Just watch out for the insane patients ("lunatics") who try and jump on your back, they are also able to jump dodge over the top of you, similar to how Batman himself does.