Melee Rampage with shivs + quick kill pistols
 Hive Scum Build - Warhammer 40k: Darktide 

Hyper-Critical kills will not benefit Hyper-Violence, however because HC only procs if the target did not already die from your regular crit, you can still have kills from crits which do continue to stack HV. 
See video below for example and proof (and pinned comment from mod creator used for testing). 
Basically HC and HV still work together in a Horde clear situation, but HC takes over when you are dealing with lots of Elites and it can even execute two at once with the first heavy attack on the Improvised Mk I Shivs. A neat combo is Heavy 1 > Light > Repeat. This way you always use the best heavy attack that can kill 2 elites at once, and then if the kills were not taken by HC your Light attack, which we only really use as a quick combo reset anw, will benefit from HV as a bonus and/or kill something to boost your next Heavy atk with HV.

So you can just spam light attacks against hordes, and though not the best it will work thanks to all the extra attack speed and Hyper-Violence. And then your heavy attacks will one shot 1 or 2 elites per crit, except Crusher but those also die in 2 or 3 hits anyway while Rampage! is on. This build has about 8k DPS on a Beast of Nurgle.

The Stub Pistols are just reliable anti-special/gunner at almost any range, with good clip size and fast reload. Unlike the Needle pistol they pierce through multiple enemies and kill instantly, also should interrupt trappers and dogs even if it does not kill on the 1st or 2nd shot.

Kalma V on the stimm and nothing else, so you can always use it to get your Ult back 50% faster and with a 45s cooldown it should always be back online before your next Ult expires. You could go Hypex  but then you are pretty much forced to stay in melee the whole time, which I don't like, rather have the option to disengage when Rampage! is cooling down, just to be safe. 
Same reason for using Stoked Rage over the other 2 unused Keystone Modifiers, it builds slightly slower, but it still builds fast (only need <10s) while it also still builds up against bosses, and then you have Adrenaline Unbound for 20s, constantly recovering your Toughness even if you are no longer in melee, with way more than enough time to build it up again before it expires.