This guide goes over the attacks used by the Great Unclean One in the Exterminatus difficulty, along with some examples of how to take it down without all too much trouble.
This fight isn’t as clean as the Lord of Change, particularly since you’re not able to whittle down the adds and deal with it alone. But since the adds accumulate over the length of the fight, meeting it head on greatly reduces that impact.
This guide goes over tackling the Great Unclean One on the Exterminatus difficulty.
The first thing to note that this fight is actually noticeably different on the different difficulties. For one thing, being in close range on some difficulties triggers the GUO’s Doomsday Bell attack, a lengthy AoE that deals light damage to the player and whose windup and looping is one of the best chances to burn the GUO down. But I’ve not found this attack to occur on Exterminatus at all.
One other major change is that the Bilesword attack is the highest priority, fires two shockwaves per swing, is repeated three times, and each shockwave deals a substantial 160 damage apiece. While easily dodgeable on their own, these add a lot of risk when combined with the mounting numbers of Nurglings or getting caught running loops.
So the strategies here are primarily more aggressive and focus on facing and destroying the GUO without retreating. However, for those who may find themselves caught without a Vortex grenade or even many heavy weapons, there is a trick that can be used to kill it without taking much damage. (This is demonstrated in the video by using bolters to do the fight in about a minute.)
I would treat the exact weapons and movements as examples; there are certainly a lot of variations that can be done.
Great Unclean One Attack List
If faced in the open, GUO will start with this attack. It does a slashing animation with its sword and two shockwaves travel towards the player. These travel slowly enough and close enough together that it isn’t too difficult to jump over both with a single jump. There will be a delay and this attack will be repeated twice more. There is a cooldown to this attack, so after the three swings the GUO will cycle through his other attacks.
There are a lot of things to note about this attack, as its one of the most impactful on the fight.
The first is that this attack will only start while the GUO has line of sight on the player. It will skip this attack if this is not the case. However, breaking line of sight will not necessarily cancel this attack, and I have had the shockwaves travel through obsticals like pillars. Because of this, I tended to allow this attack to occur and focus on damaging while jumping instead of risking getting clipped. It is possible to pay peekaboo with the GUO while using a weapon with splash damage to kill the emerging Nerglings, and I was able to score a kill while using catwalk above where the adds could not reach. But I didn’t find that to be a more efficient method.
It’s also not recommended to engage the GUO while it’s standing on an incline. This will indeed cause the shockwave to no longer be horizontal, potentially removing the option to jump. That… was a surprise.
One last thing to mention is that because of the very high damage of this attack, it’s possible that it can trigger Know No Fear leaving the player at 100+ health. Know No Fear cancels the damage when taking a fatal blow and provides brief invulnerability, and it is recharged by scoring kills. The amply spawning Nerglings mean that there’s a good chance to retreat and re-engage after using them to recharge this ability. Purposefully triggering it is certainly possible, though if you do using the last barrage of the three is probably best, since the attack as a whole is longer than the invulnerability window.
If left to complete the Bilesword attack, the GUO will next perform an animation where it opens wide the gaping maw in its gut and spews forth a torrent of these vicious but fragile critters. Attacking the GUO with a weapon with splash damage will make short work of them, as will tossing down a grenade at the start of this attack. Of the splash damage weapons, starting with the plasma works well, but you may want to switch off of it as the adds accumulate. The melta gun is the ideal weapon here, due to its extreme DPS and the fact that it can be stocked up with the Munitions Boon found on this level.
Ideally, you want to be damaging the GUO itself and killing the adds with splash damage, not going for the adds and dealing incidental damage to the boss. Honestly, I think damaging the boss and just avoiding the Nerglings is the better option if they become separated. This attack does also form an opportunity to start melee attacking the GUO if it is at critical health. Note that the GUO dying does not remove all of the adds. Being ready to clean them up with a weapon like the volkite blaster works fairly well.
Next up, the GUO will perform an animation where it plants its sword in the ground and opens up its mouth to stream a poison attack similar to those done by the plague toads. However, this attack is more prolonged and weakly tracks the player. This is not especially dangerous if you are able to move, and remains a good opportunity to continue to damage the boss.
As mentioned, this attack does not occur in Exterminatus. Sort of a pity, as it looks pretty neat. This attack has relatively high priority and triggers when the player is at close range. The GUO channels and deals somewhat modest tricks of AoE damage. So on the lower difficulties, the player might bait this attack and deal damage from range, or have to eat the ticks if using melee.
That means that there is somewhat of a more open window to perform melee attacks on the GUO on this difficulty. As long as you aren’t charging into the Bilesword attack, finishing this boss off with melee is relatively low risk. There may also be more melee focused approaches that this opens up.
Method A: Vortex Burndown
Even with a pistol start, this level has all of the tools for killing the GUO under 10 seconds and taking 0 damage. There’s a Vortex Grenade immediately before the boss, and elsewhere in the level a melta gun, a machine spirit, and munition blessing. Throwing a Vortex Grenade at the start means you only need to jump over a single Bilesword attack, and the fight will end before your meltagun ammo runs out. The plasma is also viable here with the machine spirit, but the risk of splash damage is unnecessary.
Method B: No Retreat
Move to one side and damage with the plasma during the first Bilesword attack. Throw a grenade and continue attacking during the Nergling spawn. (I like to time a reload just before the grenade toss, but this isn’t strictly necessary.) When the GUO starts its Puke Bile, approach and circle it while unloading the melta gun. The aim is to end up in an area where you can deal with the follow up attack, which might be jumping over another cycle of Bilesword slashes.
However, there’s a good chance the GUO will be at low health at this point. In this case, you can break line of sight to bait another bout of Nerglings. Toss another grenade and fire away, finishing up with melee at critical health.
Method C: Grind the Gears
Shoot the GUO with any weapon while dodging the first Bilesword. As soon as that finishes up, run down and to the right of the boss. There will be a set of deadly gears to your right. Sprint jump over them to the narrow ledge on the other side and wait.
Nurglings will kill themselves on the gears. The GUO will make its way over to you. Unload any weapon, even your standard boltgun. You’ll need to jump over the Bilesword attacks, but this is pretty trivial when it’s at that distance and the only thing you need to watch out for. Technically you may need to sidestep some poison attacks, but this is infrequent and the damage very minimal. The fight will be done in less than a minute.