This achievement seems to be glitched/unlocking for a lot of people. It was glitched for me after my first play through. The problem is completing a challenge during the prologue. I even did 130+ challenges and it still wouldn't unlock.
To unlock it, 1) I deleted my save and restarted the game. 2) I completed the prologue on EASY to ensure I would not get credit for the challenge of completing the prologue. 3) I then did 100 challenges and it unlocked immediately.
Some of the other easy of achievement not closing the game out before shutting the Xbox off. I suggest closing the game out before shutting down the Xbox.