Head up the steps and dispatch the two ogres ahead, then run up and grab the rocket launcher they were guarding. Kill the other ogre on the platform above. Jump into the water and find the lift at the side of the platform. Ride it up, and grab the shells, in the secret area. Continue on the walkway and kill the fiend you meet.
When you get to the intersection, turn right to nab a scrag. Continue until you reach the water. Kill the ogres opposite with rockets, and try to get the ogres on the platform above and to the right. Return to the intersection and choose the left path, continue towards the wall button, pressing it causes a platform to be raised on the opposite side. Work your way back to the new platform and into the next room. Retreat once the ogres see you, and kill them with relative ease as they come down the stairs. Once the ogre attack is over, enter the room and kill the fiend lurking in a right hand corner. Press the wall button, and get the red armour that is revealed along with another fiend to the right. A passage opens on the far side, go through it and nail the ogre that's waiting for you. Enter the next room and blast all the ogres there, be especially aware of the pair on a ledge high to your right. Continue around the walkway, and turn left into another hall, look into a pit below, and kill the two fiends down there. Jump down and grab the bio-siut, jump into the slime and get a box of 100 health bonus. Now get the thunderbolt and kill the scrag and ogre. Shoot the two red switches on either side of the slime pool. A Small platform will pass in front of you, jump on it, if you miss and fall in, there is a entrance in the pool to some health and a another scrag, but your bio-suit will probably already be expired. Ride the platform over to the ledge, then hop up the steps that pop out of the wall. Cross the bridge and kill the fiend and scrag waiting on the other side. Follow the corridor to the stained glass window room. Kill the scrags and continue onwards. Go up the steps and nail the fiend waiting around the corner to the left. Cross another wooden bridge, enter the cage, and press the button. You'll take an underwater journey to an ogre, kill it other scrags and an ogre on the narby ledge.
Ride the lift up, and walk into the waterway. Get the quad damage, rockets and nails. Swim up and kill two more ogres. Get the gold key at the end of the ledge above. Run back down the platform, turn around when you reach the end of the platform that had the gold key on it, Shoot at the wall to the left as you face where the gold key was, to open up another secret area. Enter and grab the red armour, then drop down and grab the quad damage rune. Jump into the water and go right, back to the base of the wide staircase. Jump out of the water towards the gold-key door, open it and kill three fiends and scrags. Now exit to the next level.