Challenge is only available during December 28th, 2014. You won't get a chance in 2015.
All you need to do is swing over to the Comic Shop, and as soon as you press
to enter it, you will go into a loading screen. And as soon as the loading screen changes to say, press
, you will get your challenge. So you never actually even have to look at the inside of the comic book. Just get to the loading screen.
The reason I put this here in so much detail is because a lot of people are having issues unlocking stuff due to the DDoSers, and I want you all to know exactly when to unlock something. So you don't wait around for 10 minutes in the shop thinking it will auto unlock.