Mama Murphy's Stimm Dealer
 Hive Scum Build - Warhammer 40k: Darktide 

Update: I've dropped Kalma V in favor of reload+crit as it's been vastly outperforming it on all fronts minus utility for certain other classes abilities. Swapped The Sweet Spot with Targeted Toxin, even though this wasn't intended to be a boss killer, it can easily outperform most due to the unique nature being able to apply full toxin stacks from such long ranges. This leads to situations where you just end up solo'ing monstrosities and extra DR helps prevent chip damage to yourself/team.

Also dropped Float Like a Butterfly and Swift Endurance to get Pickpocket which allows us to use the needle pistol as much as we possibly want to kill elites/disablers along with using the Branx MkVI Needle Pistol over the MKII to get some crowd control and proliferation using the alternate fire, this scales MUCH better the higher the intensity and creates bonkers uptime for stimm supply even without Kalma V .

Curios have been rolled to remove ability cooldown reduction for 30% sprint efficiency and 12% stamina regen. This is a game changer for actually utilizing Run 'n' Gun with Stripped Down.

This is easily one of the tankiest hive scum setups without conditionals overall outside of actually being in permanent desperado.

Mechanics

The entire setup is focused around maximizing our potential stimm cooldown with the Sample Collector passive to trigger Practiced Deployment and allow us to drop Stimm Supply as often as possible. The Blackout with Pocket Toxin + the Cadia Mk XIIIg Assault Chainsword built for crit is very good at spreading toxins and has decent horde control so you won't get overwhelmed. Branx MKII Needle Pistol allows us to inject enemies directly with toxin.

25% crit Chainsword + 20% Chem Enhanced stacks + 5% Anarchist + 5% Critical Chance Boost + 15% Vultoprene I Vultoprene II puts us at a whopping 70% crit chance potential through self buffs.  

Melee

Cadia Mk XIIIg Assault Chainsword can be changed out for any weapon, it has been the easiest application of toxin in most scenarios and the special saw attack is very useful for taking out shields on assassins. 

Improvised Mk III Shivs are very strong when rolled with Haymaker if you are more mechanically adept at the game.

Enginseer's Mk VI Crowbar also works fairly well with Bash though I prefer to use Spur V in my stimm to use the Unstoppable Force blessing.

Dumped shredder stat as it's not used that often in this setup outside of shields on assassin bosses so mobility ends up being more useful overall and the charged shredder attack still does respectable damage to the shields and bulwarks/reapers if needed.

Ranged

Use Branx MkVI Needle Pistol on armor/bulwarks/ogryns/monstrosities. I highly recommend testing in the Meat Grinder to get a feel of how much toxin is lethal for each target as this is essential for ammo management. Use the alternate fire to hit the horde between taking care of elites/specials and it creates an infinitely firing chem toxin dispersal.

Stripped Down since we do have some extra stamina there is more freedom to utilize run and gun with ranged immunity when approaching ranged situations. This is essential and allows you to stop firing lines of gunners completely safe with the curio rolls I use if need be.

Shoot monstrosities 3-4 times and then use melee crits or shoot every 3-4s to keep toxin at maximum stacks and watch them steadily die. The toxin damage does benefit from rending and can crit when stacking.

The damage roll on the needle pistol only affects the 30ish damage initial hit and can be dumped.

Blackout are for when you are surrounded by ragers/armor, need to pickup teammates/objectives, or need a huge burst of stimm cooldown from the horde via Pocket Toxin+ Sample Collector. You can use Chem Grenade or Boom Bringer over this however you may not see grenade pickups so I tend to lean towards generating ones, especially now that we can fully utilize the dispersal strain on the needle pistol.

Survivability

Chem Fortified Drop a crate the moment the mission starts and repeat to maintain 20% damage reduction

Tis but a Scratch Along with evading  and toughness regeneration rolls on curios this is a staple in our survivability as it has no cooldown and scales with % toughness gains on curios.

Burst of Energy is there to save you if you make a mistake with a relatively short CD and also scales with toughness % on curios

Precision Violence gives us good sustain from melee

Hive City Brawler provides more damage reduction so I run extra stamina to increase its efficacy window.  Add more or use less stamina, curios usually boil down to personal preference, try things out.