Combo Vet
 Veteran Build - Warhammer 40k: Darktide 

This build has been tested on Auric Maelstrom, I have no idea if it would work on harder Havocs. 

The basic idea is simple: use Weapons Specialist alongside Always Prepared to shoot high-threat targets through hordes with infinite cleave Man-Stopper buckshot (be aware that it does not overpen carapace).

When facing the uparmored Ragers, Crushers and/or Maulers the additional ranged stagger will also interrupt their actions, making it safer to shank them with Uncanny Strike and Lacerate. 

Your knife's insane mobility, combined with halfway decent movement, allows you to close in on groups of gunners and shotgunners, especially if they just got staggered by either a shotgun shot of a Shredder Frag Grenade. Just don't overextend yourself without Voice of Command. 

Combat blade's push-stab and heavy attacks will instantly kill most enemies on a weakspot hit, use them to guarantee a reload in a chaotic situation. 

Use alt-fire (the double shot) to instantly kill Mutants and chunk bosses. Otherwise it's often more practical to use single shots as they have more range -- even allowing you to hit Snipers. 

Throw Shredder Frag Grenades into mixed hordes for funny damage numbers, stagger and bleed chip. Remember that most people don't run builds that one-shot everything, so you're definitely making it easier for them to kill something.