When you find the Gauss rifle near the end of the level, do not kill all of the Ghosts that get dropped. Grab one of them out from under the Brute. Kill the Brute, then kill the Wraith. Ride the Ghost up to the barrier by Dutch. Give Dutch a rocket launcher if you have one, and he will help decimate the incoming enemies. If you were fortunate, you should have obtained a plasma cannon (most efficient, but this trick will work with any weapon) earlier in the level behind the spare tank, before you go thru the first door. It is located back in a side road off the courtyard with the multiple turrets. Use the plasma cannon with a melee attack (B) to walk/push the Ghost through the invisible barrier. This will make the end of the level quite a bit easier. You can get multiple kills by running over groups of Covenant with the LT boost on the Ghost. When everything else is mostly cleared, you can stun the Wraith with an overcharged plasma pistol, and then proceed to defeat it in any desired manner.