through the level normally until you go to the hallway that has the invisibility
power-up. Don't grab it, instead go to the next room and kill the two
When your done with that grab the power-up and go outside and grab the banshee.
Fly to the large building and you will find a big gate. Drop the banshee
and continue to the end.
(Note: Don?t hurt/kill an enemy or they will shot at you after
you attack. If you feeling tempted to kill the covenant on the bridge
don't try to kill the other banshee pilot.)