Pillar of Autumn
The pillar of Autumn is the first mission on Halo: CE and once you complete it you will obtain this achievement.
I recommend to get the most out of it is play Co-op and on Legendary, if you have a hard time on legendary just take your time, and use grenades on elites or Plasma pistol and magnum to take the shield out and head shot to kill the elites.
According to The Kudi Zone you cannot use bandana skull and unlock this achievement.
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 Bungie (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.