H40 Rashad Arbites
 Arbites Build - Warhammer 40k: Darktide 

All rounder build, stack justice by swapping between ranged and melee into hordes. Use grenades to knock bosses off edges or for general CC. Try to keep at least 1 -2 grenades handy for ambushes. Sit in your nuncio aquila radius during boss fights to clear hordes for your allies

Rashad works in this case because of the buffs to melee damage giving very high damage per swing leading to a lot of 1 hit breakpoints.

With this build I can one shot most ranged elites at H40. Use heavies to receive the  +15% damage buff then continue with lights and repeat with heavy again. Spam heavies for crusher/rager packs as well as bosses, tossing a grenade for stagger as needed (Also gives damage buff)

Slide heavies for ranged mobs along with shredder/shotgun as required. There is no need to kill with your ranged option, all you need to achieve is some level of suppression and judgement stacks.

Alternative weapons
Ius Mk IV Shredder Autopistol 

Agripinaa Mk VII Combat Shotgun 
These weapons are what I've found so far to be good at stacking judgement quickly and efficiently when shooting into hordes. Agripinna is better at longer range specialist snipes but shredder will make elite hordes a breeze.
 

If your team is good you can swap Fear of Justice for Force of Will or True Grit for better survivability when nuncio is on cooldown. True Grit helps quite a bit in H40s especially vs Twins where you may be forced to tank a melee sweep from a bad position.