Arbites Roach (Auric)
 Arbites Build - Warhammer 40k: Darktide 

Man's Best Friend + Shotguns and Mauls

I am sure there is a better build out there, but I have found this to be highly effective on Auric/Auric Maelstrom, and more importantly it's fun. I built this mainly to finish off Poxbreaker, so I leaned heavily into player damage rather than survivability. If you're having issues I'd switch to things like Imposing Force and Target Neutralized, etc. That said, Castigator's is a great oh shi- button and usually saves me when I mess up.

Switch out to whichever grenade you find most useful. I like the Arbites grenades while I'm grinding kills because they annihilate hordes, however Self-Destruct or Voltaic mines are better for support and area denial/control under most circumstances. Use whatever you want.

Playstyle

Your goal is to cause as much damage as possible. Keep running that shotgun as much as you can. Ideally everything is dead before it makes it to you. Prioritize high threat targets such as disables and gunners. Weakspot damage to Crushers and Maulers ends them fast with the Mark 8. If you do get into the mix in melee use grenades to buy yourself breathing room and then clean up. This build synergizes well with other Arbitrators and classes that can help close off part of the battlefield. You are not control heavy, but Voltaic Mandibles and Serrated Maw mean your doggo can do some serious damage to pesky specialists while you focus down other stuff. Do not be shy about using your dog, but try to keep it fairly close. Don't send it across the map to pin a specialist. Your Mark 8 can handle that, or someone else can. Man and Cyber-Mastiff provides good toughness regen, so again, try to keep it close to you.

Beyond that, just fill the gaps where needed. You can frontline effectively thanks to Castigator's Stance and enormous stagger. You can snipe with your shotgun or bolter. Have fun! Feel free to change up anything you want for your playstyle. Probably needs a lot of tweaking for Havoc.

Curios

I run the bog standard 3x Toughness HP/Tough/Gunner for all three curio slots because I'm basic. You can easily swap in one Stamina/Sniper/Combat Ability regen, but honestly I haven't run into stamina issues and Castigator's regens fast enough. Still, YMMV, use whatever you want.

Weapons
 

Exterminator

Mark 8 - Default and absolutely disgusting on long, linear corridors or Rolling Steel (the train map). It shreds everything.

Mark 3 - I take this when I know hordes will be more spread out. Mortis Trials, tighter maps, etc.

Both are viable with this build.

Agripinaa MK VII

(Flak, Maniac, Full Bore, Man-Stopper for stagger)

Also works just fine and is oftentimes the better choice. I think ammo economy is better, but you lose the cleave and personally the other shotguns are just more fun. That said, for clearing hordes or knocking things down with the slug it can't be beat.

Bolter/Bolt Pistol

(Cavalcade, Pinning Fire or Rupture for Havoc)

Work fine with the build. Bolter especially has high stagger and if you're expecting Crusher mobs or Maulers or Scab Ragers it's excellent. Also better if you adapt this for Havoc for applying bleed.

Arby Maul

It just works.

Mark 7 Tactical Axe

(Flak, Maniac, Brutal Momentum, Headtaker)

Also absolutely nuts, but better for a Terminus Build as you don't get as much stagger, which this build relies on. Still, if you want to use it it can work.

Crusher

(Unyielding, Flak, Slaughterer, Skullcrusher)

Completely viable and often hilarious. I am just not as confident in my combos as I am with the Arby Maul.
 

L1 -> block cancel -> L1 spam is default.

Push Attack -> Heavy 2 repeat is also bread and butter for hordes and mixed hordes.
Power Up -> H1 -> block cancel -> H1 for heavily armored/monstrosities.

Power up as needed and do it often for CC and extra damage.