I think I have read every topic on here discussing achievements not unlocking, and have tried all the possible fixes to get them to register. I have stopped playing because I do not wish to continue playing until it at least registers I have beat CE on legendary. Below is a list of achievements that will not register despite any number of attempts at doing them over.
Living legend
Believe in a hero
Greenhorn
Birth of a Spartan
The library
Two betrayals
Keyes
The maw
(additional misc achievements)
Leave it where it lay
Tying up loose ends
Get me out of here
This side up
I figured more posts about this may get some attention eventually. Someone from 343 please help!