This level is insane! From the start, head to just before you get to the outside section. You'll find a load of imps in a cage in the centre of the courtyard, as well as a load of shotgun marines and normal marines emerging from the four staircases on all sides. Clear the place out, and head down the at the far end of the courtyard opposite you. Kill the Imps, shotgun marines and Spectres down here, pick up the goodies, and head down the lift to kill some more Imps and pick up some more goodies. Head back up to the courtyard and down the staircase next to the corridor you started in. Down here, kill the Imps and Demons and pick up the yellow keycard. The room accessible to the side has another rocket launcher and a box of rockets, so sprint to get it, then sprint around the outside of the room and wait at the top of the stairs with your shotgun. A massive amount of enemies will teleport in on the star section, so pick them all off as they arrive. Once they're all dead, go down either of the other two staircases in the courtyard, and use the yellow keycard door accessible from them. Inside, kill the enemies and grab the red keycard, then kill the Imps and shotgun marines that come out from the wall section behind where the keycard was.
Now, run back across the room with the large block in the middle (one of the rooms off the courtyard). Go through the red keycard door, and kill all the enemies on the other side. Go around to the small tunnel at the back of the room, clear out the barrels, and throw the switch. This will trigger a side corridor to open, and the block in the previous room to lower, revealing the blue keycard and a ton of Demons. Kill them all, and grab the keycard. You can now go through the blue keycard door accessible from one of the staircases off the courtyard.
Inside this next room, kill the 4 Spectres on your side of the room, then kill the shotgun marines and Imps across the slime chasm. Use the discoloured wall section on the right to access a secret room - sprint down the staircase and across the room to the lowered platform at the end - if you're not quick enough you have to repeat it. Once up, make short sprinting jumps down the columns in turn to pick up all the goodies. Now, head back to the slime chasm room and deliberately drop down into it; follow the tunnel, and kill the Imp on the platform. Throw the switch, then head through the secret door when it opens to the lift at the end. Go up it (ignore the Imps down here; you can't fight all of them whilst on the slime) and you can go across the raised platform to the exit switch (kill any Spectres that appear).