Caustic Bomber [Havoc Ready]
 Hive Scum Build - Warhammer 40k: Darktide 

I took every damage talent that would help maximize toxin damage + the best defense talents I could reach on the skill tree.

The minimum amount of strength you will want on the stim is 12-8% from Wildfire 2 or 3 because it allows your Stimm Supply to maintain a good uptime due to only setting the stimm cooldown to 27 seconds. This is easy to reduce using Sample Collector while also provided the necessary cleave needed for your needle pistol and melee cleave. This will allow you to use the alternate fire with blow through, making it only take 5 shots to get back a Blackout nade when firing into a horde. You can go all the way up to Fury 2 and stop, as the rending does add a good chunk of damage to your toxin DoT. The Bonesaw alternate toxin should only be used on bosses. Apply toxin via your gun and melee to ramp the DoT and refresh it.

Alley Rat + Street Tough allow you to create space while kiting for reloads. Street Tough procs even when you block the damage, so make liberal use of that. Hive City Brawler works as both Damage Mitigation and Dps increase. Shove attack chains are our bread and butter in hordes and the extra damage goes a long way when working towards and extra Blackout nade. 

Stripdown gives you a ton of play making potential in Havoc due to being basically immune to ranged attacks. I do not know if it intended but you find that some ranged hits still tickle you when running. This is good as it will start our toughness generation from Tis But a Scratch. These 2 working together will help you survive and hopefully help clear elites and specials for safer revives.

Bio-chemical Warfare

This is how we are scaling our toxin's in the build

• Fury II: +20% strength and +15% Rending which translates into a lot of cleave and damage for both our Needle Pistol and our Bones Saw as well ass all our Toxin Damage.
• Virulent Strain: +10% increase to all damage enemies take.
• Toxin Mania: +5%-15% base damage for each enemy affected by a toxin. This affects our DoT.
• Punching Above One's Weight: +15% damage against elites and monsters.
• In Your Face: +10% damage dealt with Toxins when the Needle Pistol is out or up to +25% damage with our Toxins when the Needle Pistol is out and we are within 12.5 meters of the enemy.
• Quick and Deadly: +15% Close Damage [I am currently assuming that this works with Toxin based on the wording, but have no mod to confirm it's effect yet].
• Run n' Gun: +15% Close Damage when running [This affects DoT applied from all sources and is the reason why I assume Quick and Deadly works as well]

Few things to note- 12% attack speed from the two +3 stamina curios has more value to me than 17% toughness [13.6 toughness]  in Havocs as the attack speed serves 3 purposes: Faster Melee Wave Clear, Faster Brittleness stacks applied to monsters, and Faster Toughness generation via Precision Violence. This will also help with getting kills to refresh ammo and nades. We only commit a small amount to crit because it's the last bit of dps we can squeeze from the tree that matters so no 5% crit perk or Maxed out Chems [they would put at a base 50% crit] this is the main reason to take shred. Both the perk and talent are viable here but I did not have the space to include them. Shred will help us apply toxin stacks when swinging the Bonesaw with the purple coating active. 

The Secondary Melee options are either Shock and Awe or Decimator in my opinion as both give cleave, thought one amps damage but also needs to be built up.

Make use of Run n' Gun on the Needle Pistol as the increased damage applies to the DoT. I haven't tested it too much but Quick and Deadly should also increase the DoT. If not, at the very least it will increase your melee -- though given the wording the talent should 100% affect the Toxin damage.

Blackout nades thrown into hordes provide breathing room as well as a decent amp to stimm regeneration. They can also be used to keep up stacks on Boss Fights.

Obviously the Ordinance slot is open to be swapped as you have easy access to all of them, so feel free to take what you prefer.

Splash Damage is unneeded as melee kills on elites and specials don't happen too often due to killing most threats with the needly pistol. You would gimp your toxin DoT by removing one of the damage talents to grab it, so if you REALLY value it consider removing Street Tough or Alley Cat.

Cooldown Reduction does nothing for our blitz because it relies solely on the Stimm Cooldown. DO NOT use the stimm alone as it will gimp your loop for Stimm Supply. You can use it at the very beginning of a run with the Stimm Supply to get 2 quick stacks, but past that it is not recommended that you do so.

Havoc 40 modifier -> Total Toughness = 105 || Total Health  = 112