BTL/EO/Arbites Maul/Combat Shotgun
 Arbites Build - Warhammer 40k: Darktide 

"Freedom is a loaded shotgun" -KMFDM, Krank

While the Exterminator Shotgun is really fun and versatile in usage, the Combat Shotgun is very much just as good in the hands of an Arbitrator. Agripinaa Mk VII Combat Shotgun  for the slug special round to make it much easier to trigger Full Bore, stagger or knock down most Special/Elite units, and some punch through so long as it doesn't hit flak or carapace armor. Scattershot  is for shooting through hordes to hit critical targets to then up critical hit rating per target you hit and the unit(s) you were aiming at on the next shot, while also working with the keystone Efficient Killer to up critical rating even more and critical damage. Can be swapped for No Respite shotgun blessing (not to be confused with Zealot talent No Respite)  for more consistent damage to staggered and knocked down targets. You can swap the perks around to your preference, I prefer maniac/flak since the shotgun can deal with them, but you can use unyielding/maniac to deal with reapers/bulwarks/mutants/flamers/trappers/bosses, or flak/unyielding too.

"Speak softly and carry a big stick; you will go far" -Teddy Roosevelt

Branx Mk III Arbites Shock Maul with High Voltage and Execution for the blessing since you will be electrocuting enemies a lot and obviously staggering for even more damage. Typically you wanna do push attack/light/heavy/push attack/light/heavy or push attack/light/heavy/light/heavy for easy sweepers for horde clearing and heavy/light/heavy/light for single target hitting. Perks carapace/unyielding to deal with crushers/plasma gunners/maulers/bosses/armored bosses, but you can do flak/unyielding, maniac/unyielding or even maniac/flak if you'd prefer your shotgun to do the boss killing.

Talents

-Force of Will, Walk It Off, and Up Close  are your bread & butter toughness regen perks for simply using up stamina (such as push attacks), close kills and staggering melee hits. 

-Remote Detonation will be the choice of blitz in a dog build 9/10 times even with only having two charges because it's a cooldown based talent, unlike Voltaic Shock Mine and Arbites Grenade where you'd have to find grenades to restore charges in a dog based build. Note that the dog explosion radius can be buffed a bit by using a Bolt Pistol blessing Lethal Proximity. So, if you'd prefer to use a Bistol over Combat Shotgun, use the Godwyn-Branx Mk IV Bolt Pistol with Puncture, Lethal Proximity  and preferably for perks flak armored/maniacs. 

-Voltaic Mandibles Augment is a must in all dog builds for damage and crowd control, as is Concussive since that'll force weakspot hits to count as stagger hits for 4 seconds, even on unstaggerable bosses through regular means. 

-Suppression Protocols can be swapped for Suppression Force if you prefer to lower the damage output of staggered enemies over getting toughness damage reduction by 20% upon hitting 3 or more enemies at once for 5 seconds.

-Keystone Go Get 'Em!is a free 50% damage for your dog against ranged enemies and will have the dog prioritize them rather than going for whatever is closest to 'em. 

-Break the Line is the choice of ability for crowd control, as well as Kill Order and Commendation from Condemnation for 50% dog damage for 12 seconds after activation, and toughness and stamina regen based on how many elite/special/boss enemies you hit at once. 

-Hold the Line is your primary source of toughness damage resistance based on what you stagger with melee, and must be restored after getting hit with one melee attack. 

-Target the Weak for 15% extra melee damage against melee staggered enemies, Drive them Back for 75% cleave upon pushing an enemy for 5 seconds. And yes, this buff can also be applied to guns that don't explode on impact. Weight of the Lex for 15% extra damage upon heavy attack for anything you and your dog do.

-Rebreather provides 20% damage reduction to corruption, such as pox gas against your health, pox suiciders, pox mines, etc. The best part about this talent is the 75% reduced damage to pox gas, meaning even in high tiered content that gas won't do much to you so long as you're on the move and restoring your toughness.

-Zealous Dedication for extra crit chance on kill, and Soulguilt Scan for extra weakspot strength upon weakspot hit. Strength  increases damage, impact and cleave.

-Execution Order marks specials/elites/bosses that grant 15% toughness regen, 10% attack speed, and 10% damage upon kill and grants 150% damage to the dog when it pounces on 'em for 8 seconds. 

-Malocator for 50% ability cooldown reduction on marked kill for 8 seconds so that Targeted Brutality isn't too much a necessity to get. 

-Keeping Protocol to gradually ramp up that 1% extra boss damage and less damage the boss deals by 1%, up to 30% until the game is over.

-Efficient Killer for that extra 10% crit chance and 25% crit damage for 8 seconds on marked kill to ramp up the base shotgun crit chance from 5-15% and then additional stacks of crit chance from Scattershot buff.

Curios

I chose two toughness and one health curio with each having toughness/stamina regen/damage resist gunners since the majority of enemies that'll shoot you fall into the gunner category. Plasma Gunners are bugged to not be affected by the perk... for now, until Fatshark patches that. You could also swap one of the anti-gunner perks for even more corruption resistance, revive speed, anti sniper, or anti bomber. You can swap the toughness rolls for combat ability regen to lower the cooldown of Break the Line by a few seconds. On paper it doesn't sound like much, but getting it back even by a few seconds can be the difference between survival or downed/dead.

Is this high level Havoc viable?

Yes it is.