Posted by: PNut d kidnaper
FAQ: Missing an Achievement, Armor, or Other Unlockable Item? Read Here!
according to that link: The good news is that now the Spartan Officer, UNSC Spartan and Graduate achievements are all tied to the new per-playlist rating. So if you have already surpassed those milestones on a global level but never earned them, you can do it again by earning a 50 EXP level in a specific playlist.
Not true... on the new EXP system i have several playlists in the 100's. didn't work.