Hello, I beat all of the Halo MCC campaigns on legendary, through a mixture of solo and co-op and I have received the “Legend” achievement, but I have not received the Helioskrill armor and helmet. I have checked the campaigns on waypoint, and each mission of each campaign shows a legendary completion on either solo or co-op status. As far as I know, there are no other requirements for acquiring the helioskrill set, so why would I not be receiving it?
Do halo 1-4 on any difficulty (even easy) and that should work. It did for me.