"Only in death does duty end..."
Checkpoint: Chapter StartEdit
Make your way through the wreckage of Industrial Zone 82. After a minute or two you'll drop down and encounter a handful of Gretchin. Take them down and proceed.
When you reach a large archway, dispatching a lone Slugga Boy, turn around and shoot out the explosive hazard to open up a new path. In front of you should be a corpse with an audio log (Workshift Log 1/5). You can take the audio log by tapping //, or you can listen to it on the fly by holding the button instead.
Turn around again and proceed through the archway. Take out the Ork and examine the black Support Pod next to where it spawned. There should be a new weapon, the Bolter, waiting for you. Pick it up, and the door to the left of the Bolter Pod should crumble. Immediately you'll be ambushed by a pair of Shoota Boyz. Use the Bolter on the first, and then Stun and Execute the second if you've lost any health.
Go straight forward. You'll hear some Orks taunting you as you run through this corridor, but they won't attack. Keep going until you hit a checkpoint.
Move forward (notice a pattern?) and you will rendezvous with Leandros, triggering the first challenging fight in the game. During the first wave, use your Combat Knife on the ground forces. Save your Bolter ammo for the Shoota Boys who are sniping at you from the upper floors of the wreckage. There are also several ammo stashes scattered around should you need them (they glow blue and are hard to miss).
After you've cleared both floors, a second wave of Orks should be jumping down from the balcony to the main floor in front of you. Focus your attention on the ground forces, using combat rolls (//Spacebar) to dodge the gunfire from above, because you don't need to actually kill the gun-wielding Orks to trigger the end of the fight.
After a brief cutscene a wave of Gretchin swarm through the destroyed opening. If you've been paying attention this far, you'll know what to do. Proceed outside and after another conversation with Leandros, pick up the audio log (Guardsman's Journal 1/5) in front of you. After picking up the log, head right through the trenches and protect the Guardsmen on the other side. Slaughter your way through the horde; use your Bolter for groups of enemies, since its high rate of fire will eliminate them quickly. After defeating the enemies, you will trigger a cutscene.
The Imperial Guardsmen will open a trench tunnel for you. It should be easy to spot, as the blue will strike a remarkable contrast against the red and brown you've been seeing up till now. Run in a straight line past the corpses lining the walls. Go up the stairs, admiring the wreckage, before heading back outside. You should get another message from Sidonus, telling you you're near his position. Run up the stairs and follow the sound of gunfire, then approach Sidonus to trigger another cutscene.
Checkpoint: The TrenchesEdit
After the cutscene your objective should be updated: Reach the Command Bunker. At this point you have the option to swap out your Combat Knife for a Chainsword (it's a chainsaw you swing around like a sword. What more needs to be said?). Considering you'll be facing Gretchins and other Orks who like to mob you in groups, I'd recommend taking the upgrade. Swing away at the Gretchins, and take a right once you reach the trenches again. Use your Bolter to take out the Shoota Boyz hanging out at the top of the trench. Rinse and repeat as you move along the trench: focus on the Shootas first, then wail away with your Chainsword. When you reach a fork, take the right path to trigger a small cutscene that introduces Bomb Squigs. Though you won't be fighting any of these guys just yet, it's nice to know they exist. Or is it?
Anyway, once you've shot the Shoota Boyz where the bodies of two Imperial Guardsmen lie, turn around and head right to grab an ammo stash behind an explosive hazard. Turn around again and head back to where the Guardsmen were, and get to higher ground. You should see some Bomb Squigs spawning below you in the trenches. Take them out from a distance with the Bolter before dropping down and continuing along the path. Now you'll be faced with a group of Gretchins and a Bomb Squig. Shoot the Squig to get rid of the whole lot.
Run forward, and an Ork Dropship should fly by with a special present. Pick up some grenades from the ammo stash on your right and use them (or the explosive hazards, or whatever you feel like) to get rid of said present in the form of a battalion of Shoota Boyz. This should trigger another massive wave of Orks. Head to your left toward the sealed door, where a group of Imperial Guardsmen should be holding on for dear life, and take the Orks out from a distance using your grenades and Bolter. On easier difficulties you might be able to run out into the open and Chainsword everyone to pieces, but exercise caution, as you will constantly be under fire and your shields won't regenerate properly. After a while, another Dropship should stop by with reinforcements. Kill them too, and the Dropship will throw a Slugga Nob at you. This is pretty much the mini-boss fight of this chapter. Keep your distance and throw grenades at him, then rush in with a Foot Stomp combo ( //) and Execute him when the // prompt appears over his head. Alternatively, use a Shoulder Bash combo first ( //), then use a Foot Stomp combo while he's reeling. When you are executing him, you will have to quickly press the left mouse button in order to kill him. Be wary not to confuse the stun button with the execute button, because if you do, the Nob will counter with a damaging wrestling-like move. This will also happen if you fail to press the left mouse button quickly enough.
Once you've killed the Slugga Nob, run towards the sealed door where the Guardsmen were fighting earlier to trigger a short conversation convincing the Guardsmen to open said door, which leads to the Command Bunker. They do so without hesitation. Go inside, turn left and go down the stairs.
Walk forward a bit and trigger a cutscene introducing you to 2nd Lieutenant Mira. Afterwards, go to the Support Pod to the left of the stairs and pick up the Purity Seal, which should unlock Fury. By clicking both sticks (or pressing T on the PC) you can temporarily enter Fury Mode, which will buff your melee attacks and constantly regenerate your health. You can recharge the Fury Meter by (you guessed it) killing enemies. Who saw that twist coming?
Once you pick up the Purity Seal, Mira will unlock the door on her right. Follow her through it and get to the front line of the trenches. Once you've reached the barbed wire, go inside the second bunker, past where you want to go, and pick up the third audio log (Station Ventris 1/5). Then turn around and take your first left to trigger another cutscene showing a bunch of Orks deploying from a Digga. This entirely melee-based battle is designed for you to test your Fury Mode, but you should probably hold off on using it for now.
After massacring the Orks, head towards the ammo and grenade stash on your right and through the archway, dropping down into an arena-like complex on your left. Kill the first two Shoota Boyz that show up, and then another small group of Orks will spawn to your right. Kill them too. You really should be getting the hang of this by now. Follow the sound of gunfire and take out another group of Orks battling with Guardsmen. If you use an explosive hazard here, the Guardsmen will die too, but you'll have taken out so many Orks you won't need their support. Go to your right and man the Heavy Bolter, using it to rip apart any Orks in your path. You can also detach it from its moorings if you feel like it. But watch out: like so many video game mounted guns, it has a tendency to overheat, but thankfully the cooldown time if it does overheat isn't that bad. The temperature meter is just below the crosshairs. It's probably a good idea to use Fury mode to take out the horde of Orks if they're bothering you too much around the Heavy Bolter.
Once those Orks are dead, head towards the building on your right and take out the Orks attacking from it. Stay to your left and you'll see another Support Pod with some Orks nearby. Kill the Orks (for a change) before approaching the Support Pod and pick up your new toy: the Stalker Bolter, a long-range weapon which can double as a heavy-duty battle rifle or a sniper rifle. Its main disadvantage is the low ammo count (you only start with 50 rounds, compared to 360 with the Bolter). Walk forward into the ruined trenches ahead to trigger, you guessed it, a cutscene.
Checkpoint: No Man's LandEdit
Your objective should be updated: Enter the Gun Fortress. You're expected to take back a gun fortress (surprise, surprise) currently under Ork control, so Mira's regiment can have some breathing room. Head left and you'll find another ammo and grenade stash. Snatch up some of the glowing-blue goodness before proceeding forward. Take out the one Shoota Boy, then move forward to see more Shootas on the high ground. Take them out too. You don't have to use your Stalker Bolter here, but it will help a bit. Keep moving forward until you hit a checkpoint.
Go to your right and be prepared to be ambushed by some Orks with the high ground. Take them out, then head over to the ammo stash on the opposite end from where you came in. Turn around and pick up the audio log (2nd Lt. Mira 1/5) in front of you. Proceed toward the burning wreckage and take out the Orks nearby. Keep following the fire and explosions until you see a bunker. Head inside and grab the last audio log of the chapter (Guardsman Alexander 2/5). Turn around and head to your left, following the sound of the Orks.
Take out the Gretchins however you like, and follow the trail to find a much larger Ork group. There's an ammo stash behind an explosive hazard to your left if you need it; otherwise, keep advancing forward towards the gun fortress. An entire swarm of Orks will charge at you when you approach the steel door in the back, so go to work with your Chainsword. The door will magically unlock once you've killed everyone in sight, but you still have to walk up to the green switch and press // to activate the door.
Go through the door and you'll hit a checkpoint. Grab the ammo stash if you need it, head down the ramp to your left and activate the next door. There should be a Assault Support Pod right in front of you. Walk up to it and grab your well-earned Jump Pack. The Jump Pack can take some getting used to: you have to hold //Spacebar to fly, then aim the cursor pointed at the floor, at which point you can press // to hurl yourself to the ground and do massive damage to all enemies around you. You can't use the Stalker Bolter while you have the Jump Pack on, but your other guns are just fine.
This is where the game really starts to open up. Use the Jump Pack to fly over the door and get to higher ground. Stay on the upper balconies while Leandros and Sidonus deal with the Orks on the ground. There are quite a few rocket launcher-toting Shoota Boy Pros here, but you can fly out of the blast radius with a simple boost of the Jump Pack. Use your Ground Pound to deal with high-ground enemies, or you can vertically outflank the Orks, have them group around Sidonus and Leandros, and get them all in one shot. This is your time to play around with the Jump Pack and find a combat style that suits you best.
Proceed to the end of the hall and activate the door. Go through the door and get ready for another fight. This is the last big fight of the chapter, so have some fun with it. Experiment a bit more with the Jump Pack and don't hesitate to use your buddies as bait for multi-kills.
Pick up the ammo stash on your right and use your Jump Pack to outflank the Orks. Sidonus and Leandros will provide excellent distractions while you administer death from above. Once the Orks are dead, follow the electric wiring to yet another gun battle. The quarters here are a bit more cramped than in the last fight, but you can still use short-range ground pounds to stun multiple enemies at once, now just begging to be Executed. Save your Jump Pack shenanigans for the ground Orks, since the Shoota Boys here tend to have explosive hazards nearby that you can shoot, therefore there's no need to waste time charging over to them.
Once everyone's dead, head toward the flashing icon, whereupon you drop your Jump Pack, since there's no room for it inside the cramped Rok. Way to end on a low note.