To do this you need to get a squad member's XP maxed out on level 60.
If you have a character that is prestiged and the challenge has not unlocked, maybe this will work for you. I had this happen to me and figured this solution might help out some of you.
Simply load up a squad assault game with the prestiged squad member and The challenge should pop as soon as the game loads or starts. You may have to play the whole match. I cannot confirm this at this time but it seems from other people's comments below that it will pop as soon as the match loads up. Good luck and please feel free to add any additional information in the comments below and I will change the guide accordingly.