For the challenger achievement you must complete 25 challenges. I have completed 26 and it still will not give it to me. My gamer tag is abortedspawn687 and you can see on my service tag how many challenges I have completed along with my achievements where it still has it locked. I would like to get it. This is the first time a game has not given me an achievement, but I’ve had it happen to friends on other games. I’m a little upset about this, so some help and info would be nice.
Keep working on it - I have around 56 completed (across all the gametypes) and it didn’t unlock for me until last night.
I actually didn’t even notice it unlock because I was in Busy mode so thanks for your post and good luck.