Would It've Killed You To Take The Elevator - Halo CE: Beat the par time on Assault on the Control Room.
The speed runs are very easy to unlock when you set the difficulty to easy. As long as you know the path you need to run, only spend time killing any enemies that are necessary to eliminate.
The par time for this mission is: 15 minutes.
Complete the mission in under this time and your achievement should pop! Enjoy!