Hamdol's Havoc 40 Updated Rampage Low Stim Scum
 Hive Scum Build - Warhammer 40k: Darktide 

Havoc 40 build for solos and carrying. Made by player/playtester Hamdolsun.
Very comfortable and very powerful build I like to use, to the point that my friends don't want to play with me anymore if I'm on hive scum as the game gets very easy. You can finish a lot of games with minimum damage taken if you get used to the playstyle of this build.
 

PLAYSTYLE: Play selfish, push aggressively, dodge and slide often, don't revive teammates (unless its safe to do so) and keep attacking threats. Your job isn't to stay with your team, it is to remove the threats before they become a threat.
Pop rampage and try to remove elites for your team while keeping watch for any specials pushing you. With one shot to the body from the needle pistol, every special except dogs, bursters and mutants die. For the mentioned specials you need two shots instead.
For doing scans rampage gives you stun immunity so press ability before going in to not get interrupted.

For bulwarks, if there is a single bulwark light spam to open them up and inflict toxin through the shield. Heavy attacks to sides and head wil also connect (first heavy in particular). If there are multiple bulwarks do half circle dodging while slightly backing off doing H1s to hit them.

For crushers, use needle pistol to get some stacks before engaging with rampage melee. If you don't have rampage 30 stack them asap and they will wither away. If you are in rampage 4-5 heavy attacks are enough to kill a crusher so go for it.

For any boss/captain, 30 toxin stack them before engaging them and let coated weaponry keep it at 30. The extra DPS from the toxin will melt any HP bar.

For specials, pull your needle pistol and noscope them. You need to kill trappers and zoning specials asap! (flamers, bombers). Push the bursters as you have insane mobility to get out easily. If the burster is knocked down 3 shots will kill it fast enough to blow up before it gets up. 

STIM USAGE: Stim use recharges every 30 seconds, start by injecting as quickly as possible to get to three stacks and use your stimm as a quick small atk spd boost / toughness regain mid combat (slide and inject).
You lose the stim power spike for this so be careful with your rampage usage and try to maximize it.

GRENADE USAGE: 10 stacks of toxin on chem grenade means you can use it like a mini rocket launcher. Use the grenades well, try to maximize their spread and land the initial stacks on important groups. Use them sparingly, when your team is getting pushed into a corner or holding a chokepoint. You have the most important/strongest grenades in your team so push and gather all grenade boxes before your teammates do, also tell them to leave the nades for you. This also means you have to check a lot of boxes so look around.

Good luck with havoc. If you have any questions ask away using official Darktide discord to find me. My name is Hamdolsun there.