It's complicated. You play matropolis(Don't remember how to spell it), on any dificulty. play until you get to the end of the bridge but don't enter the tunnel. kill everything exept one banchee(including your marines). luer the banchee into the tunnel and lead it all the way throgh the tunnel,kill everything else(even the marines that join you). when you get to the open door after the part where you ditch you vehicle you will have to break off one wing . have it folow you out the tunnel. when you get into the curved tunnel where you get a check poin you have to quickly board it . it might not work the first time. but once you get it fly almost straight up to a big bridge like building way in the air. it's floating above a ornge cone.

Ask Any questions if you have them. i will check from day to day.