Wow, pretty confusing. Unfortunately my input probably won't clear a whole lot of this up.
While working on my Marathon Man achievement I played through 3 Meta-game levels yesterday. I was playing by myself BUT I had the Scoring set to Team and got the achievements. So, technically I was playing Solo but the scoring was set to Team. Just figured I'd mention that since people were indicating that the Achievement unlocks only in Solo Scoring.
Some of the achievements may be glitched slightly as well. For example, I played through the Heroic campaign with my wife and got all the skull achievements together except for Catch. For some reason it triggered for me but not for her. SHE was the one the picked up the skull too. Odd.
And to respond to Phrozan, the Skulls can be grabbed in both "regular" campaign and Meta-game (scoring turned on) campaign.