Walk forward while Titus gives a transmission to Leandros and Sidonus that he is, in fact, still alive. He is told to rendezvous with them at Hab Centre Andris. Continue along the sewers until you see a patch of light. That's your visual cue for your first fight. When you reach a second patch, four Gretchinz will drop from the ceiling. Dispatch them as normal. Blow up the explosives in front of you to reveal a stash of grenades. Continue on the linear path and open the door on your right.
The main difficulty in this part of the game lies in your lack of squadmates. While they were mostly useless before, they did serve as nice distractions, but now every Ork in the vicinity will focus on you instead. Just fight as you did normally, but keep an eye out for sneak attacks from any and every angle.
Once the ground has been littered with Ork corpses, continue forward and up the ramp. Take a left, and you'll hit a checkpoint.
Pick up the grenades in front of you, then backtrack to the ramp and enter the new tunnels. Follow the objective marker and a small fight will start. At first it's only a lone Shoota Boy, but then a Bomb Squig will pop in from the ceiling. Take care of them and keep moving. Open the door and go right, collecting grenades, then kill the lone Ork and move towards you. Kill the second lone Ork and move forward, hitting another checkpoint.
Follow the objective marker, and move across the walkway then right into a long, thin hallway. You'll face a good half-dozen Slugga Boyz at once, so take them out. When reinforcements arrive, do the same to them. Make sure to make good use of the explosive barrels scattered around the map, especially when the Slugga Nob shows up. Continue up and to the right, hitting a checkpoint.
Activate the door and go through it. Keep running along the path; you'll have the occasional Bomb Squig to keep you company. Eventually you'll face an 'Ard Boy and a platoon of Bomb Squigz at the same time. Just keep your distance and use grenades. When the fight's over, you will hit yet another checkpoint.
Run forward again, and hit another checkpoint....again. Look, it's a Support Pod! Run up to it and grab the Melta Gun: an absurdly powerful weapon that's almost a guaranteed one-hit kill, but with the lowest ammo capacity in the game. Test out your Melta on the upcoming wave of Slugga Boyz, and remember to group them together in your sights before you pull the trigger, otherwise you'll run out of ammo and have to use significantly less awesome weapons instead.
Once you've had your fill, continue forward, turn right and drop down. On your left is an exploding wall; shoot it and recover the Servo Skull (Medicae Station Ventris 3/5). Continue upstairs and open the door.
Continue down the cramped hallway. Once the area opens up, another fight will start. You'll come across a weapons stash on your left. Grab the Vengeance Launcher and Power Axe if you wish, then deal with the Orks and follow the objective marker to the lift. Ride it up and approach the Support Pod.
Take the Purity Seal from the Support Pod to unlock Superior Fury. Unsurprisingly, this is an upgrade to your Fury which decreases the power drain. Head upstairs and take out the Orks. Do the same with the Orks downstairs. Go forward to the objective marker and you'll rendezvous with Leandros and Sidonus at last. Objective Complete! You should get a new objective: Rendezvous with the Inquisitor. Once your squadmates have entered the tunnels, open the door and run forward.
You'll see some Orks down below. Take out as many as you can from the high ground before proceeding downstairs. Slaughter your way to the objective marker and head up the rubble. Keep moving across, and then up the stairs, and you should be out of the sewers. Finally!