There is apparently a really cool cheat in the arkham series, big head mode. Firstly, the disclaimer:-

1) I have played arkham 1-4 x3 on easy. You have to play on any difficulty but easy. I.e. I haven't verified this. No idea how i'm going to because medium is not for me.

2) there is no consistency or limited consistency between videos and code providers.

3) cant verify consistency between game stores.

On to the code

Up, up, up, down, down, down, left, left, right, left, left, up, up, down
(keyboard, either arrows or wasd keys.

Hold right+ left trigger + rotate right stick clockwise 10 times
(xbox, playstation controllers)

Supported by arkham city, origins, knight.
Can use it ingame / menu /riddlers challenge maps
Possibly need to complete story on any difficulty except easy. Can use in new game plus.

Enjoy!