Run to the right, killing the zombies there. Hide under the tree and let the various forces take each other out. When they seem done, run left and take down the banshee with the rocket launcher. Now head right to the dead bodies and gather up the loot. Grab the ghost and go forward, and watch the fight. Kill off anybody that survives. Loot them and get the medical kit and such, and the sniper rifle.
Get back in the ghost and follow the stream. At the end you have to leave it behind and jump up. There's another fight going on. Sneak around to the right during it, steal the banshee, and fly back the way you came. There are a few floaters up on the ledge, shoot them and land. When you go in there are a few more inside. Zap that final generator, and you'll go into movie mode, where you're teleported over to a Covenant ship.
Note from Guillaume:
In "Final Run", you say that you have to leave the Ghost behind before arriving to the
last big fight (where you get the Banshee). This is not true, it's actually
possible to get the Ghost over the ledge. It's not easy, but on the left side
the ledge is a bit lower and you can get it across. It's most useful
afterwards because you're less of an easy target when you go across the
Halo 2 Walkthrough
Halo 3 Walkthrough