The achievement isn't "broken" necessarily, as it can still be unlocked. I unlocked it last night with 27 challenges completed.
Basically, it may take more than 25 challenges completed to unlock, but it should eventually happen. I've seen reports of anywhere from 25-50 challenges completed before it unlocks. Just keep playing, and you'll be alright.
Here's a link to a forum post discussing the issue: Xbox.com Halo 4 Challenger Issue
If you do a Google search, you'll find several more websites where people are discussing this.