Gravemind: (roars in pain)
Cortana: "You hurt it, Chief. But not for long. We need to get to Halo - destroy the Flood once and for all."
If the Chief takes too long:

Cortana: "An explosion just made us an exit! I'll mark it on your HUD, Chief, go!"
When the Chief reaches the halfway point escaping:

Cortana: "Analyzing the route ahead... I have it mostly figured out. Just keep moving, I'll update your HUD as you go."
Cortana: "Corridors ahead have all collapsed. I'll find another way, Chief; be careful."

Cortana: "There, Chief, into the maintenance tunnel."
The Chief finds his way back when...

Cortana: "I've got a friendly contact! But who would be crazy enough to come in here?"
Chief and Cortana find The Arbiter battling Flood with a Flamethrower. The Chief assists.

Cortana: (amused) "Wait, you two made nice? What else have you been up to while I was gone...?"