I got the legend achievement but I didn't get the helioskrill armour in halo 5
Do a search in these forums. There are many solutions that will fix this for you. I think that the last one I read has you going into a custom game on MCC then quitting, this resets your stats or something to the effect. When you go back to Halo 5, your armor should be there. If this won't work, just search the forums. Good luck.
I completed all of "Legend" but the achievement didn't register missions 7- 15 of Halo 2. I checked my waypoint stats, and tried redoing the missions, but they still wont show as completed.