Mission 13: Argent D'Nur

Follow the path and drop down into a large area. Kill off the Cacodemon and some Imps, then move into a large battle. You'll DEFINITELY want to grab the Berserk Powerup when loads of powerful demons start to show up and quickly waste them with it. After clearing the area proceed.

Here, drop down one level and defeat the Unwilling. Use the Skull Switch to the right to move a large rock to the left, leading back to the upper level, this time with the door open, leading to a secret (BFG cell and fuel).

Go back down and check out the walkway to the left for some armor. A Cacodemon and Hell Knight will spawn too, so be careful. To continue, go down the walkway to the right and drop down. Some Lost Souls will spawn along the way. Check your map to also spot an AutoMap down this path. At the end, collect the Mega Health and drop down to initiate the next fight.

It's a tough fight against a lot of demons, so around halfway you'll want to grab the Quad Damage Powerup. In the beginning you can go all out with powerful weapons in combination with the Rich Get Richer Rune (infinite ammo). Stay on the move and stay far away from powerful demons. After clearing the area, head up the stairs and kill the first Wraith's Soul. Going down via the right stairs, hit the Skull Switch and check out the tunnel at the far end of the area. This leads to a secret (BFG cell and fuel), but it also leads to a Classic Map Lever (at the end of the blood trail, after climbing up). The map is inside the tunnel.

Continue out of the tunnel to reach the second battlefield. Again a lot of demons will spawn, so pull out your entire arsenal. Several Cyber-Mancubi spawn; when they do start using the Gauss Cannon on their heads from a distance. If Barons of Hell show up, move away and use a BFG round on them or preserve the Berserk powerup for this moment. When the stairs to the second Wraith's Soul are revealed, go up the left stairs and drop down the hole to find the Cosmoguy Collectible.

After destroying the second Wraith, continue through the hole on the opposite side of the area at the green-eyed skull. In this passageway use the skull switch to reveal a cubbyhole that counts as a secret (with a BFG round and fuel in it). Continue to find the third battlefield.

Again you're up against a lot of monsters, many Imps, Hell Knights and Pinkies this time. Use the Rich Get Richer Rune to wack them and resort to the Quad Damage Powerup when the Barons of Hell show up. Waste the third Wraith, then move to the marker to meet the final boss.