Timur's All Arounder (Havoc 40)
 Arbites Build - Warhammer 40k: Darktide 

This is a build that I have tried and worked quite well in high difficulty content, even after the recent nerf on dog bleeding talent (warranted but by the emperor it was hilarious when it lasted), I still believe having a dog provide more benefit in general than going lone wolf, mostly because of inherent messy chaos that is high Havoc and/or playing with few under-skilled randoms on Maelstrom. The shockmine is only going to save you if you have time to plan ahead (half a sec is still hard to come by some times), while a dog is there to stagger and knock off elites even when you are knee deep in heretic corpses with tox gas on your nose, bale flame under your feet, and two bosses on your ass. 

First, a quick explanation of what this build does: You benefit the team by providing SkullCrusher Debuff on Bosses, Toughness Damage Reduction buff and Toughness regain for all teammates (Canine Moral, Part of the Squad). You excel at face tanking gunner packs and Elite packs during your castigator stance, creating openings with your dog knockback and dog-plosion, then blast everything apart with well placed and well paced MKVIII shots. You are decent in melee, the Crusher maul with Slaughter strength buff gets you decent horde clear and control. You are quite tanky in horde, with push attack and staggering both give toughness. Your weakness is solo boss damage (but this build mitigate it somewhat so you are at least on par with other classes not specialized for boss killing), and sudden spike of damage when your Castigator is on CD. You can be ammo hungry if you are not careful with target priority and ammo conservation on high havoc.

Note on SkullCrusher: This is the best Blessing in game. It is my opinion that every high havoc team can use a SkullCrusher applier as support, and no one does the job better than Arbites, because only Arbites can apply the debuff consistently to bosses even without on CD skills like Vet shout or resources like grenade to stagger bosses (Concussive talent). What the blessing does is that if you keeps staggering an already staggered enemy, you can build up 2 stacks of SkullCrusher debuff at most, each stack applying +40% damage taken to all sources, meaning the whole team can enjoy it once you applies it. How do you stagger a boss three times in a roll? Well for you, Lexmate, it is fairly simple, just bonk it in the head three times with Concussive talent selected. First hit makes the boss count as staggered, second and third each apply a stack. I recommend installing a debuff monitoring mod so you can see this more clearly.

Now to the talent tree.

Arbitrator tree is in general excellent, there are a lot of good talents, so one has to make some choices. I have tried in quite a few different builds and have come to the following opinion.

Top Section

Must haves:

No Escape: 10% damage and movement speed on elite kill. This one is best in the row, and unlock shortest path to dog bleed talent.

Razor jaw: Dog causes bleed. It is no longer bonkers like before, but still decent damage on Boss, ogryn enemies, including Crusher, and on hounds/mutants your dog can't pounce. It also helps shortening Time to Kill (TTK) on pounced targets.

Up Close and Force of Will: 5% toughness on close kill, no matter range/melee, then 10% toughness on Staggering Melee, which with your two handed crusher and later Concussive talent is pretty much every hit.

Walk it off: get toughness back when you spend stamina. Which means in melee you will get toughness back when you push attack, which is a good chain starter for two handed Crusher. 

10% Toughness Damage Reduction (TDR): generally lacking in Arbitrator talents other than Castigator Stance. Plus we are trying to get out the top section with minimum investments, so with Walk it off really essential, this is the obvious choice. 

The 6 points above get us out of the top section with minimum investment. 

Worth Mentioning:

Target Neutralized (Replenish toughness on elite kill), Hammer of Judgement (10% attack speed while melee in horde) are also worth considering, if you decide to shift the build by taking off few points from dog related talents. 

Mid Section

Must Haves:

Remote Detonation: Dog-plosion. This is useful for staggering the boss once you have skullcrusher debuff built up on boss and unload your shotgun directly into boss head. Also useful for Bulwark wall and other heavy elite packs. 

Arbitrator Armor and Steel Plate: added toughness nodes, mandatory.

Ammo Belt: 25% extra ammo. Useful for Maelstrom, pretty much mandatory for high Havoc. Shotgun is your main damage source. Your big ass Crusher maul is for building up debuff on enemies and horde control.

Concussive: You need this to proc SkullCrusher debuff on bosses. Mandatory. This combined with SkullCrusher help solve one of the major shortcomings of Arbitrator, which is the low boss damage. Also this is your team perk basically, since SkullCrusher benefit the entire team. With a competent boss killer like a Thunderhammer Zealot or a PickAxe/Grenade Launcher Ogryn you can help kill any boss in seconds.

Voltaic Augment, Canine Moral, Part of the Squad: your dog electrocute; when your dog/you/team kills a pounced target everyone gets TDR and Toughness back; 7.5% TDR and count dog for coherence. TDR is just very valuable in high havoc, and especially for Arbitrator. This route is more beneficial for the team as a whole compare to right hand talent path. 

Worth Considering:

If doing low difficulty content, one can switch out Canine Moral, Part of the Squad to Suppression Protocol and Breaking Dissent (self TDR in horde and 10% damage to staggered foes). 

If struggling with survival, one can shave off some damage node and switch to Imposing Force (25% flat damage reduction for 5s after elite kill), it is a great survival talent. Other classes would thank the Emperor if they get a 20% TDR talent like this, while we get 25% TDR and health damage reduction. 

Keystone: All three choices, Elite, Ranged, or even just don't pick any, works. No, I am serious. It is a matter of how much you micro the dog. If you are the type of person who keeps tagging for dog, then you don't really need keystone. If you prefer to let the dog run free until you have specific tasks, then pick elite or ranged. I prefer Elite because I find it brings my dog in the middle of Melee Elite pack a lot, perfect for Dog-plosion. 

Bottom Section:

Must Haves:

Castigator Stance with Magic Bullet Refill: Just bonkers strong. Allows you to face tank lines of gunners and shot them in the face with your shotgun (your face/faith is your shield). Honestly with the Cool Down Reduction (CDR) talent below, able to pop this every 27s for 10s is what makes other classes cry nerf. 

Arbites Revelation: It is Duelist from Zealot tree. If you are not dodging a lot in high difficulty content, you are doing something wrong, so this is reliable damage buff.

Zealous Dedication: crit on kill buff. Easy to maintain damage buff. 

Execution order and Malocator: Wall-hack eyes, damage buff, CDR, all in one. Best in slot for Keystone.

Keeping Protocol: 30% damage and damage reduction to bosses, helps shore up your weakness.

Justified Measure: simple 15% damage buff, easy to maintain

Worth considering, but feel free to switch around:

Serrated Maw: Brittleness debuff on dog hit. This is helpful for softening up Crushers and bosses. Also reduce TTK on dog pounce so the good boy/girl can move on faster.

Not Far behind: 10% damage and speed when dog jumps. You want to take this when you also have selected some dog damage nods, so dog don't sit on one poor heretic for eternity. With dog bleed and electrocute nod, in practice I have found this buff to be constant during target rich environment.

Soulguilt Scan: Weakpoint damage buff, easy to maintain. This is decent for the precision shotgun we are using, and for Crusher maul's easy horizontal sweeps at head level. However, feel free to switch to other talent, say Imposing Force (25% flat damage reduction for 5s after elite kill) if you struggle to survive.

Street Smarts & Monstrosity Hunter: extra dodge and dodge window. Helps with high difficulty content, but more to enable us to grab Monstrosity Hunter (20% damage to bosses and Ogryn enemies). I usually pick these two together or neither.

Re breather: Tox gas immunity. Worth it if you see that modifier in high havoc.

Priority Endowment: one extra shotgun shell in the tube. Can be useful, but I have learned to be more careful with my shots, take a little longer so I proc Magic Bullet during Castigator Stance, and do decent even without this. Also not that needed if you pick any other ranged weapon, say MKIII Executioner Shotgun or regular combat shotguns. 

Hold the Line, Drive them Back plus either Strike Down or True Grit: this is dipping into the right most tree to boost your Melee/staying power. It will mean significant sacrifice on dog/castigator stance related points, so I don't usually go this route, but it is viable if you prefer to shoot less and melee more. Or if the Maelstrom have no ammo pickup modifier.

Weapon Choices:

Melee is Crusher Maul with SkullCrusher, just simply too good for the team combined with Concussive (+80% damage on bosses for entire team, assuming you can bonk the boss in the head thrice). It is an inferior melee weapon compare to regular one hand Arbites maul in terms of TTK on armored elite, but you got MKVIII shotgun with armor piecing shells for that, and dog helps too. Crusher maul is decent for horde control and clear with Slaughter, extra strength means you don't need much Impact talents to stagger melee elites.

Ranged you have more choice. Both MKVIII and MKIII Executioner shotguns are excellent for high level content. I prefer MKVIII when I am not sure my team is good at special/gunner quick kill, and use MKIII for easy mixed horde clear when I have a competent ranged teammate like a vet or a psyker. Slug and Dragonbreath regular combat shotguns are also both fine, though those two requires a bit more skill, as stagger is not as good as Executioner shotguns. Bolt Pistol and Revolver are fine up until Maelstrom but will struggle in high havoc. Bolter is not exactly needed for this build, since you have SkullCrusher to help whole team do more damage to boss, the pull up time is so long that sometimes after you applied SkullCrusher your thunderhammer Zealot killed the poor boss before you even pulled out your bolter. 

Curios:

3 Toughness or 2 Toughness and 1 Health if you struggle surviving. For minor perks go for Toughness and CDR, then a mix of gunner/sniper damage reduction and stamina regain. I find gunner resistance less important the more I play the game, since you have so many ways to suppress gunners and dodge shots. Stamina regain on this build is very good, because how often you use Crusher push attack, and the more stamina you use the more toughness you get back because of your talent.

Tactics:

Use MKVIII shotgun on tougher enemies, including Crusher. Pop a special AP shell in there, then usually 1~2 regular shot they are dead. Same for Bulwalk, as long as you see an opening of their shield (can be baited by having them start melee). Use shotgun on elite packs, while popping castigator stance. Use big bonk stick on boss head for debuff, and on hordes/chaff, as well as squashy elite/specials that you can easily reach so as to conserve ammo. I see so many trigger happy Arbites just shoot anything that moves till chamber empty then starts to melee everything. This is not wise. Only use your ranged weapon when the situation is right, like there are a wall of elite coming in, or a far away sniper, or you already have applied SkullCrusher debuff to boss and now it is time for party.

Shout out to Foralza, who saw me in action with this build doing well and encouraged me to share the build with the community. I would have never thought of writing a guide like this if not for you.