When you find the Gauss rifle near the end of the level, grab one of the Ghosts that get dropped and use it to kill the Wraith. Ride the Ghost up to the barrier by Dutch. Give Dutch a rocket launcher (if you have one) to watch him decimate the incoming enemies. If you were lucky 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. 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 much 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 defeat it in any desired manner.