Instructions
Locate the Merc that enters the room after your conversation with Dacher. Zap him from a distance, and then run up and stomp him. Head through the large door and into the corridor. Proceed to the right, and you'll see a cutaway scene with the Mech.
Continue up the corridor. Shoot out the lights to keep the Mech from targeting you. Use your EyeShine to find your way in the dark. Sidestep left and use the crates for cover as you advance. Turn the corner and run into the Waste Disposal Room. Head up the ramp, and pause by the pillar to replenish your health. The Mech will follow you into the room. Run up and activate the trap door switch, dropping the Mech into the trash room below. Riddick will follow.
Activate your EyeShine in the dark trash room. Face the main wall to your right, and you should see a trash wall on the left. Get between these two walls, and then run forward to the large cylinder on the left side.
Keep your eye on the Mech as it begins to move toward you. Aim for the Mech's head, and use your Assault Rifle to take a few shots through the gap. Sidestep to dodge the Mech's attack. When it finishes firing, step back in and shoot at its exposed head. Continue dodging and shooting until the Mech falls.