Halo Walkthrough: Upstairs Downstairs
When you're brought up in the gravity well, follow the nav point to the big room. The bad guys are 'gathering bodies'. Lovely. Keep going, you'll end up going down into that room. Grab an overshield to one side, kill everybody and keep going. Go down the hole, watch them fight, and go forward. You'll hit a BIG fight, and another shuttle bay.
Master Halo Walkthrough