The Legend of 117
To obtain this achievement you must finish every Halo 4 mission on Legendary difficulty the list of missions is below.
If you watch the video ProAceJoker does a speed run of the missions if you are looking for tricks to do legendary, also he can make them easier and you can fight less enemies. Remember that he is doing a speed run so unless you want to get the Are you Joking achievement you don't have to go as fast as him, it isn't required for this achievement.
Dawn (1st mission)
Requiem (2nd mission)
Forerunner (3rd mission)
Note: The mission achievements have been really bugged so if your achievement says "Done.... Unlocking" your in the same boat as basically every gamer. I still have 1 mission achievement unlocked a week and a half ago and it still says "Done.... Unlocking" don't keep attempting to unlock it because it won't happen. Just pray Microsoft or 343 (whoever has to fix this will) which with the amount of people its affecting, I can assure you something will be done.
Update 12/09/2014: Seems as though they fixed all level achievements, either it will unlock on you randomly (some say) or you can definitely unlock it by replaying the level. It won't unlock immediately, but at a random time, most likely you will have it in 24 hours.