From what i have seen so far of 5 beta looks good but i havent seen the helioskrill armor yet it was meant to be in the beta so i did all 4 campaigns on legendary in mcc is it in the beta or did they take it out.
If you actually did all the campaigns legendary it should have unlocked. It might have been a bug, though.
But how did you beat all 4 campaigns on legendary? My friend and I spent a few hours just to beat like 2 missions in CE.
I did 1 and 3 coop and 2 and 4 solo i dont have the beta atm i just havent seen anyone use it in videos i watch.
I think you have to beat each campaign individualy in 3 hours on legendary, but I could be wrong.
I did every campaign in solo and unlocked, Legend achievement. Then I got the armor in the beta. Make sure you have ‘‘Legend’’ achievement unlocked
https://www.youtube.com/channel/UC21z65wTqPZhqdp6Xs5OIaw, my Youtube “channel”
I’ve got 3 short clips of gameplay rocking the helm only. I prefer others but since it is so rare apparently, rock on!!! 
Double check that there is Legendary coat of arms beside every level and cutscene. Double check you have the achievement unlocked for Legendary completion where needed in each game.
Lastly, if you used some the x0 mulitplier skulls, that may have prevented the unlock… This I admit I am unsure and you’d need further clarification if the first two checks are OK,
What I found for me is that levels I did not complete in one playthrough, ie, not quitting and than starting from last checkpoint later, did not count… Grrrr. Luckily I knew which ones I needed when I figured it out during achievement hunting.
I just really like the helioskrill design but havent seen any one use it in youtube videos but thanks for the help:)
I have it and I actually uploaded a few clips if you wanna see it
yt channel is my account and gt