You just have to FINISH all 50 of the arcade challenges. To make this even easier, this can be done on ANY difficulty so if you want to breeze through it like I did, set the difficulty to 0% and you could probably finish all 50 in about 1.5 - 2 hours.
If you win all the challenges (on 0% difficulty it is harder to lose than to win), you will get the following achievement: