Checkpoint: Chapter StartEdit
The first part of this chapter is an "arena" style battle with several waves of enemies. There will be no checkpoint until all (four) waves are dead, and some players have had mild trouble with this part on Hard difficulty. Bringing along a Lascannon and a Melta Gun should make it easier.
Approach Nemeroth (surrounded by a bubble that prevents you from reaching or harming him) to trigger some dialogue and the first wave, consisting of a mob of Bloodletters. This part is quite easy, as they have no ranged support and you can freely roam across the area, thinning them down with pistol or Kraken Bolter fire. Grenades are very effective here as the Bloodletters will bunch up while chasing you (just don't shoot them too much since they'll teleport and spread out).
When the daemons are gone, it's time for the second wave, this time with several Chaos Space Marines ("Tactical" variant) supported by Renegade Militia. Quickly take cover behind the boxes located on the left just after the arena entrance; this position provides some cover from both sides, and ending up in a crossfire will kill you on this wave. If you get a chance to snipe a CSM, take it, otherwise, chill out behind the boxes and take out the Militia first, then kill the CSM one by one with the Lascannon or repeated Kraken Bolter headshots (this is quite safe from cover as long as another enemy isn't shooting at you while you do it). If they toss grenades at you, try to avoid it without becoming exposed to both sides and return behind the boxes ASAP.
The third wave will be tough, consisting of two Tainted Psykers - which can kill you in seconds on Hard – and a mob of Bloodletters to deny chilling behind cover this time. You must get rid of the Psykers immediately or this wave becomes very hard. Start by moving behind the same boxes as in the last wave, and move left so that you can see the elevated area on Nemeroth's left, but the boxes block the right side. The Psykers will appear on the elevation to both sides, and you can now snipe the other when he appears, while the right-side Psyker can't see you and the Bloodletters aren't there yet. When the first one is down, peek around the boxes on the other side and snipe the other one. After this, the wave is cake as there are only Bloodletters (use the same tactics as the first wave to take them out). If you're having trouble, don't be afraid of activating Marksman Mode to snipe the Psykers – it should be recharged once you've killed off the Bloodletters.
The fourth wave is all melee, including two Chaos Space Marines with two-handed Daemon Mauls (the "Champion" variant) and another mob of Bloodletters. Hopefully, you have the Melta Gun, as it makes this otherwise infuriating stage a cakewalk. They no ranged attacks at all, and repeated Melta blasts will just murder the whole mob. If you have any grenades left, feel free to use them all now. If you don't have the Melta Gun, you can snipe the Champions with the Lascannon as they cross the barrier or activate Fury Mode to Execute both Champions right away.
When this last bunch is dead, you finally get a checkpoint so if you screw up beyond this, you don't need to do the "arena" part again.
Checkpoint: Over the EdgeEdit
While you've beaten his lackeys, Nemeroth himself has had enough time for a partial transformation into a Daemon Prince. Fortunately, he's an all Quick-Time Event boss, so just keep an eye on the button prompts, and you'll be fine.
Both Nemeroth and Titus start out in freefall, you can shoot at Nemeroth with your Kraken Bolter if you want. After a bit of time, Titus catches up with Nemeroth, and the QTEs begin. Hammer away on the Ranged Attack button to pummel on Nemeroth, and the Evasion/Jump button to avoid his counterstrikes.
After a while, he'll catch you in a vice grip, and you must hammer away on the Melee Attack button to get out of it. Don't take too long, you'll take health damage every few seconds while in the grip. Once you've broken out, Nemeroth will knock Titus away, and the freefall portion repeats, only with Nemeroth firing Warp energy at you this time. Keep moving around, and eventually, another QTE sequence will start up.
It's a repeat of the previous QTE sequence, but with one difference at the end. Once you've broken out of his grip again, one last prompt pops up. Hammer away on the Melee Attack button to crush Nemeroth's skull with your bare hands.
Once you've accomplished the final prompt, relax and pat yourself on the back, as it's all cutscene from here onwards. Congratulations on beating Warhammer 40,000: Space Marine.