Strap a Shock Mine to your Mastiff
 Arbites Build - Warhammer 40k: Darktide 

Mastiff Version of:

https://darktide.gameslantern.com/builds/9f3e12e1-f1d8-4c94-ac75-3675a20cb9f4/traded-my-dog-for-a-shock-mine

Main Changes:

• We trade Walk it Off for Man and Cyber-Mastiff, 5% Toughness regen with good uptime if you don't send your Mastiff too far ahead.
• Take Remote Detonation instead of Voltaic Shock Mine, because that's the build :P
• Lone Wolf is gone, use that one as your flex-point in case you want something else. I personally chose to pick up Retaliatory Force
• We forgo Strike Down and Street Smarts to pick up Voltaic Mandibles Augment and the 20% Ranged Critical rending bonus from Prosecution Blow

These changes are mainly to accommodate the Mastiff play style, you can also keep most of the original build, except for Lone Wolf.

Weapon: 

The Shot Pistol & Riot Shield deals pretty insane damage to Bosses, under the condition that they are staggered/electrocuted:

1. Always take Full Bore, it's very reliable due to the low spread
2. Get either High Voltage or No Respite as the second blessing on your Shot Pistol
3. Let the Mastiff pounce (+trigger remote detonate). 
4. Then unload into their Weakspot and see the HP bar melt away.

High Voltage is only worth it if you specced into Voltaic Mandibles Augment. You can technically save that point since Remote Denotation also staggers bosses, but if you are out of charges, then miss out majorly on potential damage. The electrocution from the Mastiff pounce is a lot more consistent and does not rely on a cooldown.

The Riot Shield has the added benefit of being able to trigger Drive them Back's Cleave bonus. And it's very safe to take out hordes of gunners, since you can Block their shots while shooting back.