To obtain this achievement you must finish the mission forerunner on any difficulty which is the 2nd mission on Halo 4 campaign. If you play the game or Halo 4 campaign from beginning to end you will obtain this achievement as it isn't missable.
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.