From what I have heard, the Scarecrow challenge is pretty weak. Its probably not on the disc, its more than likely the same as all the add on challenges, the Crime Alley and the Joker one for the PS3, downloadable.
With the Crime Alley challenge, you get a code which you then have to enter on the Playstation store to redeem code and download the pack. Its probably the same for the 360 Live as well.
Each code is one time usable so the only way to get it is to get the game from the shop in question, or search eGay to see if someone is selling an unused code!
