Take the banshee down to the tunnel. Open the door, but it sticks, and you have to walk through. Shoot everything you can with the banshee before you abandon it. You'll be in a big, open room. Cross the bridge to the right, grenading enemies as you go. There's a medical kit and rocket launcher on the other side. There are big blast doors to open. There's a long quiet corridor, and then a second door. Open it to head back outside.

Master Halo Walkthrough