Halo Walkthrough: If I Had a Super Weapon
You're now outside again. The entrance to the control room is at the top of the pyramid structure. Kill the group in front of the door, or if you're fast, just race like mad for the Banshee and jump in. The pyramid they're talking about is a SHORT pyramid, not the really tall thing. Fly down to it, shooting the Banshee that chases you.
Note that there are tons of monsters on the other side of the door you see. Open the door and then race for the turret to blast them all with unlimited ammo. When you've killed them all, get into the Banshee and fly to door #2. Open it and then hop into the banshee to kill anything. You're told that enemies have been eliminated. Hmmmm. Door #3 leads to a movie about "this is it". She rambles a bit and says you have to stop Keyes.
Master Halo Walkthrough