Another member of the forum suggested that by redoing a campaign mission outside of the LASO playlist will unlock the achievement. I tested it out today and can confirm it worked. I believe this fix only works (it may not work for you, disclaimer) if your achievement is stuck in the “Done Unlocking…” nonsense. I believe the error of this achievement is an issue on 343’s end, otherwise I do not think this achievement would have re-unlocked for me.
What I did to do this achievement somewhat quickly.
- Select Halo CE Assault on the Control (Recommended co-op to help alleviate the effects of Iron) - Select Legendary - Turn on all skulls except for bandanna and grenade pinata(the one that leads to extra grenades from melees). You should have 12 skulls turned on. - Follow this video’s instructions.You do not need to do this level in any amount of time, so ignore the speed run part of the video, but doing the easy trick in the video does make it a breeze to finish the mission.
If this does or does not work for you, please post below.