In this chapter, all hell breaks loose as the Daemons invade, and some unfinished business is finally finished.
The chapter starts with Titus discussing his plan to use the Titan Invictus to sever the Orbital Spire. Once he's done talking, move forward and head down into a pipe. When you go upstairs, hit a left to shoot out an explosive wall and pick up a Servo Skull (Worker Nathaniel 4/5) then take the right path. Another explosive obstacle on your right reveals some more ammo and grenades. Walk forward towards the Support Pod.
Pick up an Iron Halo from the support pod. This upgrades your shields. A bit behind and to the left of the support pod is another Servo Skull (Inquisitor Drogan 3/10). Follow the sound of gunfire and you'll see more Orks and Daemons fighting. They'll keep killing each other off no matter what you do, so jump in and start your usual murdering. You'll also be introduced to some new units, like the Chaos Space Marine. Once everyone's dead, follow the objective marker upstairs and you'll hit another checkpoint.
Pick up weapons and ammo and continue forward into a very short cutscene. Move forward again.
You'll be ambushed by some Renegade Militia and Chaos Space Marines. Use your long-range weapons to deal with them. After you've advanced forward, some more Bloodletter Daemons will spawn. Kill them too. Head downstairs, and aid some Imperial Guardsmen who are being attacked by still more Bloodletter Daemons.
The Orks and Daemons continue their fight in the next room, but the battlefield is considerably larger this time. You won't really have the ability to manipulate the battlefield and help one side or the other win, so just run in and slaughter everyone like usual. Run to the lift as pointed out by the objective marker, and activate it.
After a brief conversation where your squadmates angst about the absurd difficulty of taking down agents of Chaos, continue your usual march forward. An Ork force will attack, so shoot them until they don't get up. There's another Servo Skull (Guardsman Alexander 4/5) to the left of the next checkpoint.
Pick up weapons and ammo, and you'll find even more Orks to kill. Continue forward and kill the 'Ard Boy you meet on the stairs. Proceed down, killing more 'Ard Boyz and Rokkit Boyz. Once everyone's killed, Mira will point out a lift. Find it with the objective marker. Go up the lift and across the walkway. When you hear a Guardsman's message, turn right to find a Servo Skull, then continue along the main path until you hit a checkpoint.
Head down into the wreckage and grab some ammo. Get the Plasma Gun from the support pod, and grab another Servo Skull (Transmechanic Petronius 5/5) to the left of that. Continue forward, and you will be given a new objective: Defend the Outpost. Kill all the Orks in said outpost.
Continue to kill Orks. Once you've killed enough Orks, a checkpoint will activate.
Mira warns that defenses at another nearby outpost are weak. Move out and get there. It's now your job to power up the sentry guns so they can kill the Orks faster. Head down into the trenches. To your right is another Servo Skull (Guardsman Alexander 5/5).
Continue along the trenches. Kill every Ork you see and another checkpoint will activate.
Keep moving and get to the sentry guns, killing everyone you see. Activate the gun and another checkpoint will activate.
Now you have to Return to the Manufactorum. Keep moving and killing, following your objective marker. Continue to walk forward and open the door. SURPRISE! We finally get a boss fight in Warboss Grimskull, who apparently survived his last encouter with Daemons.
To defeat Grimskull, shoot him until he dies. Okay, maybe it's not that simple.
Grimskull's main gimmicks are grenade spam, and a headlong bullrush that does a lot of damage. When he's on the ground, he'll throw about four or five grenades at once at you, and sometimes a flashbang for good measure. When he charges, you need to get out of the way, but when he charges into the wall instead you have a couple of seconds to return the favor. Once he's taken enough damage, he will jump into the rafters and start firing rockets down at you. Damage them and continue to shoot at Grimskull.
If necessary, refer to the Tactics section of Warboss Grimskull's own page for more information.
With Grimskull out of the way, take the lift back up and rendezvous with your squad to end the chapter.