Good Dog Havoc 40
 Arbites Build - Warhammer 40k: Darktide 

Weapon Choice
Rashad is going to be used 99% of the time, it can be used against bosses, but the shotpistol will delete bosses thanks to specific combos.

Shotpistol is our main boss dps weapon, but we are taking flak damage so we can also take out any gunner, or similar high priority target that our dog isnt currently taking out. but you should always be marking targets for your dog since that seems to double the dogs already fast movement speed towards that target.

The arbites shock maul is also very strong, the rashad was always my favorite weapon in the game, but im a meta slave and dueling sword and knife where just too good not to use, but since the arbites has neither of them, i can go back to my baby that i havent used since launch. 

If you want a more versatile ranged weapon, use the boltgun. you can special snipe better, and it has better special and elite group killing power overall than the shotpistol, but you are also loosing a lot of effective ammo. since most of the time when you are going to be using the boltgun, you are holding down the trigger deleting anything in front of you, thats a lot of ammo being deleted very fast, so its easy to run out. then combine it with other teammates using ammo, there is overall less ammo to go around, so the shotpistol with its more targeted use, is better imo.

Boss Combo

If you are missing any ammo, reload, if not, shoot 1 bullet then reload. while you are doing that mark the boss for your dog to pounce it, while the dog is on the boss, hit your grenade button and detonate it, then unload with the shotpistol. then repeat until boss is dead. thanks to all the bonuses we have from the tree, this will deal an immense amount of damage. 

Alternate skill choices

If you want to be more tanky, you can take forceful over execution order (the bottom purple trees) You can also take the maul and shield, but if you do that i would swap the ranged weapon to the spearhead boltgun for higher burst and more overall versatile use, since your melee option is losing some strength for increased defenses. 

If you want more boss damage you can take out suppression protocols in the middle right, and keeping protocol in the bottom left, and swap it to street smarts, and monstrosity hunter in the bottom middle section.

Unleashed brutality or Go get em

Go get em sounds better on paper, but in reality you should be hitting your marked key through hoards when you hear/see a high priority ranged target in the first place, making this pretty pointless. now if you dont want to be marking targets, and just let the dog do its own thing, take it. but you are losing a point to travel, so somewhere you are going to have to give something up. 

Unleashed brutality on the other hand is just slightly more useful, but neither of them are incredible. if you are getting jumped by 10 ragers, now its 9, but it would be 9 even without the trait, while that is true, the increased damage means that the dog will be able to kill it faster, then take another rager out of the fight faster, and i think over a run, that is more impactful than your dog being 10 miles from you taking out targets that you can tell the dog to get yourself. 

Overall its a toss up, so what ever you value more, take. but again, you are losing a point by going with unleased brutality.

Castigators stance or Nuncio aquila

I think nuncio aquila is a bit of a bait. the bonuses look good on paper, but when your skill tree does the same things, but better, there is pretty much no point in it. The only thing it offers in reality is a smaller damage reduction compared to castigators stance, increased revive speed, which is a complete bait stat, you are either free to revive someone, or they are unsavable, 30% speed will not change that, this needs to become a auto revive, then it would be useful. fear of justice on the other hand is quite the good bonus when you think about it, you are greatly reducing your chance to be hit in the first place, and if you happen to be hit, its even further reduced. but overall, your tree gives you so many of those same bonuses, that are activated constantly, not just in the area of your ability, that its just redundant. castigators stance on the other hand, 80% damage reduction, full stop is OP, then combo that with the damage bonus when killing elite and specialists, its the clear winner.

But i will say, nuncio aquila is not completely useless since it applies to your teammates, not just yourself, but if you want to help your teammates in a more impactful way, take the shock mine. imo this ability needs some changes and buffs for it to be reasonably taken over castigators stance in any high level play. if fatshark somehow sees this, give it a auto revive, change the attack speed buff to 15%, and make it so we can either ping to move it, or that it follows us at a slow speed, like walk speed for example.

EDITS: 

Weapons
So now that i have done more testing with the different melee's and ranged weapons i have some slightly different view points. The combat shotgun is crazy on this class, it feels like a literal sniper, and is now my preferred ranged weapon for the arbites. It still does not have the same damage as the shotpistol for boss damage, but with this build, with every ranged weapon, there are missions where i am doing more boss damage, than my teammates overall damage. The combat shotgun with the extra ammo just feels better, you can put down far more elites in a short time than you can with the shotpistol, and you are only sacrificing maybe 15% boss dps, for double the elite killing power. But at the end of the day, any ranged weapon is good on this class if you leverage the damage bonuses from the tree, all of them can 1v1 a daemonhost, so just pick what you enjoy. 

And after using the arbites shock maul more and trying out more trait combos, it replaces the combat axe as my favorite melee weapon, the attack speed is so fast, no need to weave in heavy attacks, just spam light attacks and watch everything die. Very brain off enjoyable gameplay from it.