What the heck.... I'm having the same problem.... They need to fix this.
Could it the next update
I didn't get it until I completed like my 30th challenge. So basically keep doing them and eventually it should pop. Keep an eye on your waypoint page if you want to keep track of how many are done.
I say this is crap! I normally can give two SH!TS about a challenge but since I have ever single Halo achievement this achievement will drive me nuts being bugged! Total Crap!
I just unlocked it at 27 challenges...None being from the campaign. I still haven't gotten my achievements for finishing mission 8 and the campaign.