Did the Halo 2 campaign coop (oh boi what a grind) with a friend and looking back on now I did not get the achievment after on steam or xbox.
Says that I did all the missions on my own on heroic (which I did aswell) but seems to say that the halo 2 campaign was unplayed in coop.
Prob no fix but if I was going to do it againn (because I hate myself) is there any way to prevent this from happening?