Lihoe's Scumlinger build. (Havoc 30+)
 Hive Scum Build - Warhammer 40k: Darktide 

This build will be updated as I continue to play and get a better feeling on it.

After Rotten Armour finally got out of rotation, we can play Scumslinger freely, and it's plenty of fun and effective with the proper team.

Suggestions/Alternatives:

• If there's a Veteran in your team, and he's not running Krak Grenade, feel free to swap Chem Grenade with Boom Bringer.
• The Catachan Mk III Combat Blade is mainly for mobility, you can swap it with Improvised Mk I Shivs if you desire.
• Feel free to play around the stimm lab with whatever feels better for you as long as you can keep Chemical Dependency up. In which case, if you are struggling with keeping it up, replace Maxed Out Chems with Calling for a Time Out for extra survivability or whichever talent fits your playstyle best.
• You can use the Catachan Mk I "Devil's Claw" Sword as a defensive melee alternative.
• The Curios aren't set in stone, just don't waste a perk slot for Damage Resistances. We are avoiding damage here.

HOW TO PLAY:

Your main role is to be the damage dealer, boss softener/killer and special killer. Most of the time specials aren't that far away of your effective range, in which cases you don't have much of an excuse to not kill them. However, there are things you should keep in mind:

• Use your stimms as early as you can to max out Chemical Dependency.
• You're made of paper, but pack constant reliable damage against everything that isn't a Crusher or a Scab Captain shield. As such, your mobility is the main thing keeping you alive, aside Enhanced Desperado's passive ranged immunity, so ALWAYS be dodging. Talents like Hive City Brawler, Burst of Energy, Tis but a Scratch and Chem Fortified help with chip damage, don't rely on them to tank any damage. The best way to not die is to simply avoid damage altogether, and you should be good enough at that.
• Always stay with your team. It feels incredibly redundant to say, but too many players apparently are too eager to go on solo adventures and inevitably die. Your team is your best defensive resource, and you're their best damage source, so you should be complementing each other and covering your weaknesses while enhancing your strengths. The only times you should stray from your team, be it for ammo if you somehow ran out, quick loot check or dispatching a ritual from Heinous Rituals. In such scenarios, you should give your team a heads-up or ask for quick cover, otherwise, just bite the bullet and blame yourself.
• If no one is gonna use a grenades pickup, dump whatever you have left before picking it up. It's not mandatory nor necessary, but if the opportunity arises where it's worth doing, just do it. It can certainly help if the team is not returning to that room or the box is before a drop off.
• Although it's not obligatory, the Vraks Mk V Infantry Autogun as of now it's the best mark for the build, as you can get a consistent uptime with Unload.
• If a Captain or the Twins spawn, your main role is to keep the horde and specials at bay as your team deals with them. If Rinda Karnak or Scab Captain are targeting you, just avoid them and DON'T drive them into your team, you can just hold sprint+w in circles to have Rinda off your mates. If Rodin Karnak is targeting you, you can just poke him at range while cleaning hordes and specials as long as you keep dodging his shoots or got Enhanced Desperado up.
• When running Boom Bringer, don't waste it on bosses nor hordes. It should be a panic prevention button, as it's too slow to pull up in an emergency, or to help a teammate revive. But if you're running Chem Grenade, you should use it to prevent hordes swarming an area where a boss currently is or gonna be fought in, help secure a revive/objective, ease up a room with too much pressure or prevent such pressure. Their usage ultimately comes down to your game sense, as situations can vary a lot.
• Avoid giving your Ogryn backshots.
• Take full advantage of Enhanced Desperado's ranged immunity. As you can run to Scab Gunner's faces, draw their aggro and easily kill them if your team is being pinned down, or prevent such situation from happening entirely. However, if there are Reaper, they should be top priority alongside Specials in general.
• Don't shoot at Poxburster near your team, only try to kill them if you are SURE you can safely dispatch them before they can pose any threat to the team.
• Avoid picking up ammo if you have teammates that make good use of it.