Looks like you must complete the following: -
Wolf Creek - 62 Challenges
Paracel Storm - 54 Challenges
Coyote Falls - 58 Challenges
There are a total of 522 possible stars, but as you only need 350 stars for the Supernova achievement you can get away with just getting 1 or 2 stars in some.
So if you plan on the completion and are getting 2 stars average, it makes sense to just move to the next one etc
Looks like they must have patched it - I just loaded challenges up and achievement popped!