Performance Officer (Support Cop)
 Arbites Build - Warhammer 40k: Darktide 

Intro

The Duty of the Arbites in Grendal's war on Atoma is not just to keep an eye on him and investigate his true motives. They are also there because, despite some impressive feats from the penal legions, the war goes on and the Imperial tithe has yet to be restored. Among the Arbites forces sent alongside the rejects are the Performance Officers, those who's duty it is to make sure the criminals they fight alongside (as well as any fellow Arbites) serve the Emperium to the fullest extent of their ability. 
 

Build Purpose and advice

The purpose of this build is to provide toughness regeneration to your allies and debuffs to your enemies through your Nuncio Aquila, Cyber Mastiff, Terminus Warrant, and weapons. I will go in depth on each below.

Nuncio Aquilla

The Nuncio Aquilla is the biggest support tool in your kit. Use the Aquila during hoard ambushes and other extreme situations that need an area of protection and an anchor that your allies can go to. You can also send it to a downed ally take advantage of the revive speed boost. As long as you keep getting those Terminus Warrant stacks to activate Obstinate, you should always have one ready for when your team needs it.

Cyber Mastiff

Part of the squad serves 2 purposes. it means that your teammates will benefit from a maximum 5+ stacks of coherency bonus, and allows you to send your dog to help any allies struggling with elites and give them an extra coherency anchor while the dog mauls it's victim to death. It might help to use text or voice chat to let your team know this, as otherwise they may not even notice it. Theres also the benefit of the miriad of debuffs that the dog appies to enemies it's pounced, which can make dealing with Ogryn and Monstrisities leagues easier.

(I might update this build later after I test whether the bleed and brittleness together kills enemies too quickly to make the coherency work) 

Weapons & Terminus Warrant

Your number 1 priority should be to keep staggering enemies to give them stacks of Suppressive Force and just generally help with crowd control, for enemies that you can't stagger, you should hit weakspots so that they count as such. The best weapons for this are the shock mauls (both Arbites and generic versions), the Crusher, and the Maul and Sheild. 

Ranged weapons are your fastest way of getting those stacks of Justice. and are best used for either starting your Obstinate chain or for helping your allies with Terminal Decree.

It's easier to list the Ranged weapons that won't work as well than the ones that will, so I'll do that. You should avoid using the shotpistol, the Exaction Mk VIII shotgun, the Agripinaa Mk VII shotgun, and the Graia Mk VII Autogun. For weapons that I'd recommend? I'd say the other shotguns are your best bet. They can hit a lot of enemies in a single shot, benefit from the increased cleave of Ranged Justice, and the Mk III exterminator and Zarona Mk VI can get a lot of weakspot hits in the right circumstances. The other weapons that weren't mentioned are still good though, and I'd recommend finding whichever one you can do best with and going with that.