Havoc 40ish Stimm Crate
 Hive Scum Build - Warhammer 40k: Darktide 

Your primary role is Corruption Cleansing teammates (Especially good for puddles, drop a crate and your team can walk through them for 15 seconds). And specialist picking with the Needle Pistol, since you have Pickpocket you don´t have to steal ammo from the veteran as long as you can kill elites or specialists, and even if you do run out you can kill a special in melee and still get the 20% of reserve back.

You can switch between the Chem Nades and Boom Bringer if you want but I prefer the RPG for a quick elite clear if needed since you usually don´t need crowd clear if you have a psyker. Also the Bonesaw has great CC with the Toxin and Brittleness with the secondary mode. Swap between them depending on enemies, Maulers / Crushers / Scab Ragers go with Purple. Poxwalkers and other low armor / no armor enemies go with Toxin for faster cooldown on stim regen.

Try to pick off any enemies that you can and don´t be afraid to drop an "empty" crate (Stimm on cooldown) as it doesn´t reset the entire length and gives you a stack into chemical dependency. (toughness damage reduction.)

Make sure to almost always be dodging inbetween and during shots / melee hits to avoid taking unecessary damage, if you start getting shredded drop your medcrate for a quick toughness boost and slide into cover if you need. 

ALSO DON´T BOTHER USING YOUR STIM ON YOURSELF (as in not dropping the crate), IF YOU DROP YOUR CRATE IT BENEFITS YOUR TEAM AND NOT JUST YOU.