I completed every halo campaign on legendary with no skulls and i still havent got the legend achievment
I can see your problem, Forerunner (H4) and Regret (H2) have not been completed according to waypoint. Sometimes it just takes a while to update, other times you have to play those missions again. I remember that I had to play Infinity once on Legendary and once again on easy before it would count. Also make sure you’re connected to Xbox Live when doing this achievement, as I mentioned before the whole thing is dependant on waypoint.
I hope this helps.
EDIT: I forgot to mention that you can tell what ones aren’t completed by checking your service record on your waypoint profile