Checkpoint 1: Chapter StartEdit
With your new objective (Stop the Ork Assault) in hand, turn right and head down past the wreckage until you walk into a cutscene. An Inquisition servo skull bears a message from Inquisitor Drogan asking the Ultramarine squad to help him at Manufactorum Ajakis to help protect some sort of experimental weapon from falling into Ork hands.
Go through the open path near the burning wreckage, slightly to your left. You'll find a switch next to some shining ammo boxes. Hit the switch and the bridge in front of you will lower. Your squadmates will point out a large crane that the Orks are operating, and to your left are a group of about seven or eight Orks infighting. Now's your chance to lob a grenade in there and get rid of as many as you can. Prepare for a fight with significantly more than eight Orcs. It's nothing too taxing compared to what you've faced before; deal with them as you see fit.
Once the ground forces have been dealt with, snipe the Shoota Boyz ahead with your Stalker. All the way to your right and behind you, there should be another Servo Skull (Guardsman's Journal 3/5); pick it up. Turn around and continue up the ramp, which should trigger another fight. This fight is in a rather rectangular area with decent maneuverability, so deal with the ground Orks however you wish.
After a while, a ramp will descend revealing Ork reinforcements, including a Slugga Nob. One of the best ways to deal with this crowd is to stun the Slugga Nob and engage it in a melee duel. When you've mashed // enough, you'll kill the Slugga Nob and stun all the Orks in the immediate surrounding area. Slaughter as many as you can while they're stunned.
Once your enemies are defeated, head up the ramp the Orks conveniently left for you...except, of course, it's an ambush. Thankfully said ambush consists of one Shoota Boy, so snipe him and then continue down the walkway past the electric generators. Once you get up the stairs, you should meet another pair of Shoota Boyz. Dispatch them, then get ready for another Ork army (plus a lone Slugga Nob) on your right. Thankfully, the Orks are in something of a chokepoint, so here melee is extremely effective for killing multiple Orks at once. Once they're dead, move forward and drop the crane on the Orks to stop their assault. Objective complete! You should receive a transmission from Corporal Antioch, saying that you can reach a service lift to get to the Titan Manufactorum, and then hit a checkpoint.
Video (NOTE: Chapter 5 starts at 3:32):
To the left of where you dropped the crane is another switch, which will lower another bridge. Hit it, and a squad of Shoota Boyz and Bomb Squigz will attack. Dispatch of them, and descend into the trenches. A large group of Gretchinz will run at you. Group them together around the explosives barrels in front of you and shoot it to kill many of them at once. The Gretchinz will just keep coming, and some Shoota Boyz will try and get you from the high ground. Focus on killing them first before wailing away at the helpless ground forces with your Chainsword.
Move up and right and you should receive another message from Corporal Antioch, telling you to hurry. The rest is straightforward. Keep moving, slaughtering any Shoota Boy or Slugga Boy in your way, and follow the objective marker. Head down some stairs to hit another checkpoint.
You'll know you're heading the right way when your squadmates note that the lift is close by, and you'll receive another transmission from Antioch, who is sending the lift up to you. There should be an explosion at the far end of the room, and more Orks will pile in, mostly Slugga Boyz. Lucky for you, this is the last major fight of the chapter. It's almost over!
The first major wave is all Slugga Boyz. Eventually some Shoota Boyz will join the fray, and then finally Slugga Nobz. You'll the know the fight is over when you see the objective marker, so follow it and take the lift. A short conversation with Antioch follows. Once you've stepped off the lift, the last Servo Skull (Encrypted Report: Inquisitor Drogan 1/5) should be right in front of you, so pick it up. Walk forward to end the chapter.