How do you get these missions

[xWolfex]

What achievement do you have to do to unlock the flaming horns mission. I am at the last chapter of the game i beat 1 through 7 so do I have to beat the game or is it an achievement title that unlocks this mission?

[beatrixkiddo]

You have to play Multi-mode to access Flaming Horns.

[Volcompat321]

and remember, you dont have to have other players in your party to do ad-hoc missions.

[Echelon]

although some missions may require at least 2 people, like "Cost of reasearch"