I had a few achievement that were bugged. The Dave and Bob achievements, and I think the no vehicles on Winter Contingency was bugged (I will explain why I think that was bugged but not sure).
This worked two different times for sure for two different bugged achievements.
Unfortunately you need an Xbox console. I got this solution from True Achievements and its based on a user’s solution that goes by Vrruumm.
I am parpharsiing his fix and adding a few detials to make things clearer.
On the Xbox console: set console to offline. Delete local save (go to halo, menu click, manage game and addons, then saves). Load up the game, then redo the bugged achievement. Go back into Xbox settings and get the console back online. See that it connects to XBL. Close out the game. Loaded the game back up (I had to do this twice, both time I did it. maybe I was too fast. I don’t know. What you want to see is a sync screen pop up that will give you TWO choices. Load the OLDER the save. The original poster of this fix said his achievement pop during the sync. For the I’m Sorry, Dave achievement, I just started playing Winter Contingency and spamming the zoom in button and playing through the mission. It finally popped when I got to the very first group of enemies. I have no idea if it was a time thing, or spamming the button, or getting far enough in the level. Same for What About Bob? It did not pop until I had repeated the achievement. I think They’ve Always Been Faster was bugged for me, but maybe I did it on the wrong difficulty level the first time. I don’t remember. I did that at the same time as the I’m Sorry, Dave achievement. I later went back to PC played some, tried the Bob achievement, did not work. Went to console and tried, did not work. Did the fix again, and it worked. So I don’t know if you need to do the fix each time. That sounds nuts, but maybe do all your bugged achievements in one go on the console before going back to PC version. I think the problem lies there.