I beat the campaign solo on Legendary, got the achievements, but no Mark VI armor.
While I was playing, my internet cut out but since I was playing solo I didn’t think anything of it. Then I beat the game, got the achievements, signed back into live, and had no new armor.
According to Waypoint, I haven’t completed all the missions on Legendary.
Do I have to play these missions again? According to the FAQ I do, but I have the achievements, and I did it the way it said to do it (only using “Save and Quit” and “Resume Campaign”). This was rather difficult for me, and I don’t really want to do it again.
I think the achievement is valid proof, no?
