Grab the double-barrelled shotgun, step onto the lift, and hit the red switch. Shoot the two grunts before finishing the descent, then run forward to blast the enforcer, you can easily avoid it's shots if you remain at a distance. Get the armour, and enter the passageway. Shoot the grunt that appears to the left, then pause at the intersection. Enter the right hallway and kill enforcer at the bottom. Continue and shoot the grunt guarding the switch at the end, press it and return to the intersection, choose the left route. Shoot the grunt. Shoot into the shadows under the walkway on the right hand wall, it should open to reveal a secret of the red armour. Grab the nearby nailgun, kill a few more grunts and press the other button. A wall has been lowered by the first button. Shoot the explosive posts to kill off most of the grunts waiting there. Make your way up the slope, obviously killing all the grunts and enforcers you encounter.
You find your self in a large hall with a central platform over a slime pool. A platform extends, shoot all the enemy and walk down the stairs on the left, and hop over onto a platform for a secret area. Take the lift back up, jump off and get the quad damage, and waste the enforcers that have appeared. The third secret area required you to stand on the central platform, in front of the button, jump into the slime, and swim for a entrance in the wall opposite. Keep swimming ahead and you'll enter the third secret area, now go through the portal and pick up the blue armour, kill a nearby grunt, then drop down to a ledge below, and walk around to the 100 health bonus. Return along the ledge to another portal. Follow the ledge right to the portal back to the beginning of the level. Make you way through the complex, and on through to the passageway past the central platform, kill the emenies you encounter, and make your way to the lift at the end. Turn left and shoot the grunt guarding the gold key card. Grab the card kill the other grunts. Run back over to the lift and shoot the grunts and enforcers on the other side. Enter the crate structure through a passage on the other side. Jump onto the low brown crate, and make your way up to the highest row of crates. Turn left and shoot the wall to open a secret door. Continue through the secret passage and get the 100 health bonus. Jump out the window on to the platform below to the left, another secret area containing a quad damage is under the platform. Get it and run past the crates into the corridor beyond, open the door and kill everything. Keep on going and you find the portal to the exit.