Doom/E1M8: Phobos Anomaly

This is the final mission of Knee-Deep in the Dead, and it serves as a "boss" mission. To start with, hit the switch on the wall in front of you to lower the wall, revealing a load of Demons penned-in by explosive barrels. Whack out the chaingun and keep firing until the barrels detonate and all the Demons are dead. Cross the room and get on the rising platform at the end, to get a Computer Map and a bunch of stimpacks. Go left from where you started after getting them and go down to the door. Look on the right-hand side for a discoloured wall - open it for a Soulsphere. Head through the door and kill the two Demons in the side rooms. The left hand room contains 60 shotgun shells and a shotgun, and the right hand room contains 200 bullets and a chaingun, useful for what you're about to face.

Head down the long corridor to the switch at the end. Press it to lower a lift. Upon getting to the top, step off the lift and two doors will open, revealing two Barons of Hell! These are by far the hardest enemy you will have come across yet, taking numerous shotgun blasts and relentless pounding from the chaingun to die. Thankfully, though, if you've played through the episode in its entirety or have a save handy, you'll have a rocket launcher too. You'll be stuck with a shotgun and chaingun otherwise. You will want to first of all barge past the ten Spectres hiding in the star-shaped arena to get the Partial Invisibility Sphere behind all of the explosive barrels, this will help by reducing the chance of the Barons hitting you and thus buying you a bit of time to use the chaingun to blow away all the Spectres that swamp you. After that, switch to the rocket launcher and pump rockets at the Barons until both are dead - it will take several direct hits to kill them, but it's faster than using the shotgun or chaingun.

When they're both dead, the walls around the arena will lower, revealing a large open space beyond. At the north end of the star is a structure located in the open space, so drop down and head towards it. Hit the switch to raise the stairs to an ornate demon symbol at the top. Step onto it to be teleported... to an ambush! You don't survive it.

Secrets

 * 1) A supercharge is located in the discoloured wall on the steps down.

Trivia
Normally, Barons of Hell are immune to each other's ranged attacks. However, if one hits a barrel, catching the other Baron of Hell in the explosion, they will start attacking each other with their melee attacks, eventually finishing off the other.

When the arena wall lowers after the Barons of Hell are killed, you can use the wall to jump across to the exit directly without waiting for the stairs to raise.

Deathmatch
There are no additional items in deathmatch. However, you can use a switch located in the right hideout of the Baron of Hell to reach the other half of the map.

When the Baron of Hells were killed (if any), the arena walls lower. If you manage to get knocked outside, you won't be able to return unless another generous soul either frags you or lowers the lift between the two halves.