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I completed all the campaigns (excluding ODST) on legendary and still didn’t get the armor, i don’t know if I have to now beat Halo 3 ODST on legendary, or complete a certain playlist(s). But my best guess is that I have to complete every mission on solo, since I played a couple on co-op.
What dies the achievement progress for LEGEND say. What about each individual level in the actual game? You have completed them all one way or the other so may just bugging or a delay.

Try power cycling your console and see if that helps up front
I checked his service record for halo MCC there's a few missions of the campaigns he has not completed on Halo CE to Halo 4

On Halo CE he has 4 mission's not completed on Legendary solo and 2 left in co op

On Halo 2 he has all missions completed on Legendary Solo

Halo 3 he has all missions completed on Legendary solo

Halo 4 he has 2 missions left he has not completed on Legendary solo, its 3 mission's not completed on co op on Legendary.

Once he has completed these missions on solo or co op then he will unlock the HelioSkrill and all sorted :)

(ODST does not count towards the Legend Achievement)

(Sorry for double posting here trying to help this person too)
If I do sum of the campaigns on solo but do others on co-op would it still count toward the achievements or do I gotta stay on one path (path being a full solo play through or a full co-op play through)
It dont matter how you complete them on solo or co op they will still count towards the achievement :)
MCC legendary definitely the best way. Once you get past Halo 2 Legendary the rest should be quite a bit easier
It would for sure take less time to just do the Legendary campaign route in MCC than to invest the playtime in H5 to unlock it thru REQ packs, unless you get very lucky with RNG and unlock it unusually early.
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