A Dog Like No Other ( Havoc )
 Arbites Build - Warhammer 40k: Darktide 

Build Alternative :

1. If you aren't in need of Rebreather ( no pox gas spam, blight spread, dodging rotten armor explosion and/or twin pox bomb, etc etc ) then you have multiple great options. Hold the Line is a great defensive option considering the special attack of the Agni Mk Ia Shock Maul and the dog's jump / Remote Detonation. Ammo Belt will directly impact your usage of range weapon and your team's ammo economy. Blessed Armament will give a better usage of your range weapon on Castigator's Stance stance activation / duration but keep in mind this isn't a build made to be played as range. Target Selection and Not Far Behindwill almost always be active in horde considering how strong your dog is, buffing even further your Castigator's Stance dps. No Lenienceand Prosecution Blow can be considered, but there uptime won't be nearly as much as any other perfect listed above.
2. You can take the mark Munitorum Mk III Shock Maul rathen than the Agni Mk Ia Shock Maul, it'll change the attack patern but will have the exact same effectiveness, The idea being relying on push attacks and sweep attack against hordes, and special + overhead against bosses and carapace armor enemies.
3. You can swap one 5% toughness with 20% resistence against gunners if you desire. You can also swap the last 20% resistence against gunners with 5% toughness. Go with what you prefer.

Strength :

• You barely have to rely on your team, in both build and mindset, you're extremely selfish. You give 2 things, amazing and consistent melee damage, and extremely high tankiness. Paired with a partner you control ( the dog ) that does a lot of damage, your clutch potential is most likely one of the highest in the game. 
• You break easily captain shields with the Agni Mk Ia Shock Maul and can follow it up easily with overhead heavies ( gaining a lot of damage from Concussive and Skullcrusher ) that does an extremely important damage to bosses. 
• Have an extremely strong way to deal with monstrosities. Castigator's Stance while pinging the monstrosity, followed by Remote Detonation on pounce, the rest is ether overheads heavies, or headshot with Pinpointing target active. all of that, of course affected by Keeping Protocol 
• Your dog in this build might be the most important part. It provide your team with effectively a 5th teammate. It will rush and eliminate gunners, trapper and the like on it's own ( and of course can be guided when necessary. ) Will provide insane momentary horde clear with Remote Detonation or do admirable damage to monstrosities on top of giving them a lot of different effects.
• Between Serrated Maw, Razor-Jaw Augment, Voltaic Mandibles Augment and Bloodlust, your dog will apply bleeds, britleness and electrify anything it jumps on and/or pounce. Those effects are applied the moment the dog hit the target, which means that boss will be under all of those effects, before you use Remote Detonation.
• Considering The raw ammount of toughness the arbites have and regenerate with this build, paired with the fact that your dog can save you from other bad doggos. You only have 2 real threats in the form of overheads ( crushers and maulers as you do not have True Grit and trappers ( that you can also ether shoot and one shot, or ping for your dog.
• Castigator's Stance make you almost invincible on it's own, which let you afford a much more aggressive playstyle, ether away from your team, or next to them to give them a strong frontline.
• You have infinitly regenerating grenades ( blitz ) with Remote Detonation which allow a much better grenade economy for your team.
• The fact that you electrify targets so easily makes High Voltage basically always active against target that matters.
• Have the possibility to kill far away targets even without ammo by controlling the dog

Weakness :

• Even so your dog can be used when you do not have ammo anymore to actively kill away targets, your Judgement Mk IV Subductor Shotpistol and Riot Shield is an important part of your boss damage and utility. Taking that into consideration, and even by being careful of your ammo usage, you will need to pick up ammo, even with a Veteran Survivalist on your team. ( you simply do not have any way to generate or get free ammo. )
• You do not provide your team with anything that actively help them survive; No Chorus of Spiritual Fortitude, no Voice of Command, no Nuncio-Aquila, no Loyal Protector, no Telekine Dome, etc etc. 
  You do provide damage but less than a Psyker or a Hive Scum. You do provide high consistent stagger, but other can also do that. The only thing you provide that cannot be proided by any other means, is a very powerful 5th teammate, the dog.
• Simply letting the dog run arround will not make this build shine, having a precise understanding of when to ping and how to use Remote Detonation is crucial for a very effective gameplay.

Conclusion :

You aren't the most meta, be it as a build ( do not have a Break the Line, Forceful, True Grit build ) or as a class ( most comps would prefer something else. ). that doesn't mean you aren't working extremely effectively.
Extremely strong on your own and providing from my own experience, the best clutches you can get, it's a fun build and way to play the game that can change drastically your experience and your team composition without making it actively week. 
A lot of fun ! but as always, that is subjective. 

I encourage you to try it out and tell me what you think ! ;)