Please, forgive me my English 
I’ve finished Halo mission in Halo CE over week ago but achievement didn’t pop up, I did it twice once solo and once in co-op, both runs on legendary. On achievements list I just see it’s 100% progress and description “Done! Unlocking…” How long should i wait to get it? I’m affraid that I will not get this one for completing Halo CE on legendary 
I’ve had the same issue. All the campaign completion achievements for Halo: CE unlocked for me except “Silent Cartographer,” “Truth & Reconciliation,” and “The Library.” It’s been about a week now, and they’re still sitting there with the “Done! Unlocking…” message. I don’t know how or when this might be fixed. I was able to still get the achievement for completing the game, though, so that’s something, I guess.
I have 5 that have been “Done! Unlocking…” since 11/11, including Halo and Pillar of Autumn. I’ve contacted Xbox Support, but they keep linking these forums, which have minimally mentioned achievement fixes.
My “Halo” achievement showed the “done, unlocking” thing for a few hours and then went away without actually unlocking. I did another run through and it went back to saying “done, unlocking” for about an hour and was gone again. The first time I finished it (without a 0.00x skull on) was the 22nd and I’ve yet to actually get the achievement. I don’t think I’m ever going to get it.
On a side note, I watched a terminal video last night while I was playing, and it showed me one that I’d already watched. I’m also playing halo 2 at the same time so I’m not sure if they just have the same video twice (lazy) or if it actually just showed me the wrong video. Either way, just another disappointment. (it’s the terminal on “assault on control room”)