To get the achievement, you must start the "Silent Auditorium" mission.
Once in the mission, you are outside the Auditorium. You must then eliminate all the enemies to open the door and access the Auditorium in less than 3 minutes.
A simple technique is to pick up a heavy weapon (preferably a
rocket launcher) from a forward base
and start the mission with the Skull
Bandana activated to have
unlimited ammo.
Here is a guide to help you:
If you haven't unlocked the
Bandana Skull yet, here's a guide that might help: