After the cutscene you'll see a Ghost to your northwest. Get in before the Special Operations Elite in front of it does. Once you're inside, look straight ahead. There will be a hill and a way to get on top, go about a 4th of the way up and wait for an Enforcer come straight out of the large opening in the wall. It'll go straight if you stay still. When he starts to turn go up and make a left turn into the door. Follow the path until you reach the 2nd door. When the music starts to play, go right. A Sentinel will attack you but ignore it and drive past the pillar in front of you. Watch out for the Flood-controlled Scorpion tank. Make another right down a ramp then make a left and go through the door ahead but wait for a Flood driven warthog to come through first.