Part 28

Begin the fight with a Pinkie and numerous Hell Knights, Revenants, Imps, and Hell Razers. A string of about 4 Cyber-Mancubi will appear, followed by a final encounter with a Cyber-Mancubus and a Hell Baron spawning simultaneously. A Berserk Powerup is available. Use the chainsaw if low on ammo.

After the fight, gather supplies near the waypoint (a Wraith's Soul). On the left stairs, look for a large hole in the upper half to find SECRET 13.1, COLLECTIBLE 13.1: "Chaingun Model, Cosmoguy", by walking under the stairs.

Interact with the waypoint and follow it to the next fight.

You will teleport to an area with molten rock. As you walk up the tunnel, look for the third Skull Switch on the right. Interact with it, then return towards the entrance to find a newly opened alcove, SECRET 13.4, containing Meta Health and BFG Ammo. Activate Rich get Richer before taking the health.

Mission 13: Fight: Wraith Fight 3: Snipe a Hell Knight and an Imp before jumping down, if possible. This fight involves Hell Knights, Pinkies, and Imps. Towards the end, a Cyber-Mancubus will appear, followed by 2 Hell Barons, which are your best chance to complete the Hold Still Challenge using the BFG.

After the fight, gather supplies and interact. Follow the waypoint to the final boss fight.

Mission 13: Boss Fight: Spider Mastermind: This is your battle with Olivia Pierce. Unlike other boss fights, there is no checkpoint (an auto-save point you restart from on death) in the middle.

Phase 1: Dodge the Rocket Volley by moving continuously in one direction. Avoid the Back-And-Forth Laser by jumping over the low swipe and crouching under the high swipe.

When the Spider Mastermind reaches 50% health, she adds the Toxic Pond attack. Stand on the sprouting pillars as the ground is damaging. The first time, this is all that happens. Subsequent times, Olivia will concentrate on the pillars, and spikes will emerge when she jumps, requiring a double jump to avoid.

Around 10% health, she becomes more aggressive, firing weak lasers in all directions and launching damaging cubic missiles that must be dodged.

At 0% health, she will stand up, then flop down and become Glory Killable. She will not recover from being staggered.

You will receive COLLECTIBLE 13.2 "Spider Mastermind Model, Goldguy" and 6 Weapon Upgrade points.

Watch the cut-scene and credits.