You move down into a firefight with lots of grunts. Move along further - the roof will cave in and you'll see a giant spider creature up there. keep shooting your ground enemies. You're in the "C3" area.

Keep shooting. Soon you get to a red green light blinking area. There's a downed shadow here but you can't fly it. Head on through the door - "the covenant must be trying to regroup - don't let them".

There's more tunnel flying. keep working your way down. checkpoint

You hit an area with a downed warthog and ghosts. There's another red / green light pair here.

You get a cut scene of the big nasty. Now you get dropped off a big tank. The rest of the platoon is dead. The chief is going to blow up anything bewteen him and the prophet of regret.

Halo 2 Walkthrough

Halo 3 Walkthrough