Crate Spam Scum
 Hive Scum Build - Warhammer 40k: Darktide 

Very comfortable H40 build. Swap between Blackout and Boom Bringer depending on comfort and if anyone else in team needs nade pickups (such as Frag Bomb Ogryn or another Scum with Boom Bringer). Stim cooldown is 15s max, with Sample Collector you can drop a crate into a H40 Horde (100 chaff + elites) and the 7 stacks of toxin from Booby Trap will kill enough chaff to instantly bring your stim back online, thus allowing another crate drop. Lowest time I have gotten is 7s due to 5s to explode + 2s to tick twice. 

In H40 the 7 stack of toxin are enough to kill everything aside from Bosses/Ogryns/Maulers/Ragers/Muties/Scab Plasma/Scab Flamer. For everything other than Bosses/Ogryns, it leaves the survivors in easy 1-shot range. Paired with Splash Damage & Coated Weaponry, elite packs melt before you. It also pairs well with Hyper-Critical as the first tick of 7, then 6 stacks significantly chunks their health, bringing them within the threshold for a 1-shot. As this build has 92.5% crit in melee (see breakdown below) and you should generally aim for weakspots due to the high finesse on the Shivs + Precognition adding an additional +45% finesse damage, this is proccing constantly. And with Punching Above One's Weight, 1-shotting Ragers is fairly common (especially on Heavy 2)

In a clutch situation, you can kite and drop crates every 15s or less to thin the pack while focusing on priority targets with your gun/melee. Dropping the crate also instantly restores 56.25% toughness thanks to Chem Fortified + Barrage I and you'll always be getting a nice 20% Toughness Damage Reduction from the same node.

Only thing to beware are Reapers, aside from Blackouts, you don't have a way to quickly stagger them before they melt you, so I recommend picking off from afar with your Needle Pistol. Crushers and Bulwarks can be annoying so engage by shooting some needles/throwing some daggers into them first, drop a crate before impact to stagger and get some toxin on them (as well as make space if there is any chaff swarming around), then aim for the heads for that sweet finesse damage. 

Crit Calc:

Base Scum: 10%

Anarchist: 5%

Critical Chance Boost: 5%

Chem Dependency x4: 20%

MK1 Shivs: 12.5%*

Standing Crit Chance: 52.5%

Float Like a Butterfly: 20%

Riposte: 20%

Total Crit Chance in Melee: 92.5%

* The MK1 shivs seem to have a built in 12% crit chance while swinging them. You can test this in the Psykhanium even if you’re on console by taking the Channeled Devastation talent and swinging once to watch your crit chance jump up and down on swing.