Prolly already been answered, but do you have to use the blind skull for it to count?
LASO Legendary All Skulls On. (ALL) Not sure if there's a separate challenge for Solo or Co-op.
There is. You either have to do it ALL on Legendary solo or you have to do it ALL on Co-Op mixing/matching will not unlock it on Waypoint. --- GT: Incredigasmic Finished Pokemon Black National Dex @ 85 Hours and 15 minutes on 04/08/11!
But now an even bigger question: Does this also work for campaign challenges?
No, it does not. You have to play the entire mission for the Challenges to complete. --- By the edge of those wilds, warriors have charged. But now the years have passed, the generals have left and the grass grows in the armors hollow on the hill.