First kill the ogre and grunt, then collect the nails from the water. Go through the doorway, up the stairs and kill the grunt and ogre on the bridge to your left. With ogres try to get close enough so that they don't try to hit you with the grenade, but not close enough, that they can hurt you with the chain-saw.
On the bridge turn to the right and kill the grunt, there is some green armour in the corner. Now get the double-barrelled shotgun from the other landing and continue around. Using the double-barrelled shotgun, try to kill the ogre before it has time to do any damage to you. Press on around the trying to avoid too much damage from the nail traps in the walls. Kill the second ogre and the grunts. Then look for a red shape on the wall, shoot it and if you need it, get the yellow armour by jumping up the steps.
Go up the stairs where the grunts had been, knights will be released to your left as you enter the room. As they only have swords, shoot at them while retreating to keep some distance between you. Once they are all gone, press the flashing pentagram switch on the wall. That reveals a wooden bridge at the end of the room, on the fair side is the silver key. Get it then turn around and kill the grunt on the far side. Continue around, killing grunts, look for the pillar with a stone sticking out near the bottom. Push this to reveal the quad damage secret.
Grab the quad, turn around and head down the stairs. Kill the grunts and open the door that requires the silver key. Kill the two ogres waiting for you, step over the stone on the floor that reveals a quot;fiendquot;. If you still have quad, blast it while retreating. If not, jump into one of the portals that are revealed and shoot it from above.
Once it's dead, get the other two secrets on this level. Return to the yellow armour. Jump into the water and swim towards the wall with the red shape, the wall slides up revealing a secret slope. Go up it and grab the health and walk through the portal to the green armour you saw on the bridge earler. Jump into the water again, turn left and swim through the gap and up the slope for another secret area. Continue on and return to the area that the fiend and ogres were in.