From StrategyWiki, the free strategy guide and walkthrough wiki

Note: There is effectively no extra health if you are playing on Ultra-Violence or Nightmare; the most you can get is green armor. If you get injured, you'll have to wait until the next map to recover health. Consequently, this is one of the most difficult levels in the game.

Just outside the entrance, you will be flanked by two shotgun guy groups. Quickly kill them and fall back - they will teleport closer and attack. Avoid the armor shard pickups for now.

Head across the slime pit and flip the switch to open the door to the second room. Get the green armor, and collect the shards that you skipped earlier. Save the medikits until you are under 50% health.

You may want to get the rocket launcher - that is available by lowering the lift in the left slime pit.

Head into the second area, and collect the key. You'll cause monsters to teleport to different areas of the map, including a Baron of Hell (on HMP or harder). You can inflict some damage with the chaingun, but your priority should be survival. Head back to the first area into the red key gate.

Enter, kill the imps behind the platform, and get the blue key. Go to the exit to end the level, picking up the medikits if there are any left.

[edit] Secrets

1. If you fall into the first slime pit, use the lift to get out. Behind this is a rocket launcher, but the secret is a hidden door to your left.
2. Past the Red key gate, use the red torch on the left. The "NIN" floor will lower behind you. Step onto the floor that is not NIN to get the secret. Walkiing off the front of the lowered NIN floor will seal off the room - press the newly visible switch lower it again.

[edit] Deathmatch

• Partial Invisibility in the slime-pit secret
• Supercharge and kevlar armor is available from platforms from the slime-pit secret's teleporters. (The teleporters determine which platform you enter - the west goes to the west platform, etc.)
• Plasma gun is on the lift in the second room.
• Berserk is in the exit.