Invincible DOOMguy Gun Psyker
 Psyker Build - Warhammer 40k: Darktide 

This is the first build I made all by myself and my my, it's insane.

This build can carry the entire team easily - tested in Auric.

VERY ammo-intensive build, but I have no trouble finding sufficient supply regardless, even when teaming up with multiple Veterans.
You are fast, so you cover more ground and find more ammo.

The strategy of this build is to maintain maximum Scrier's Gaze Uptime for damage and critical chance and shoot marked enemies for Disrupt Destiny bonuses. This makes your damage output absolutely insane, makes you fast and also invincible to ranged attacks. That said, I can play Darktide like DOOM - running around frantically and shooting anyything that moves.
 

Critical chance makes this build really  shine - the insane fire rate of VRAKS mk V makes it easy to keep generating lots of critical hits by shooting your enemies, which make you invincible to ranged attacks thanks to Empathic Evasion.

Weaknesses of this build are mainly:
1. Crushers - I usually use  melee heavy + Scrier's gaze to kill them in approx. 3  heavy headshots. But generally, anti-armor builds in your party should focus them, so that you can focus elites, specials, bosses and hordes.
2. Armored Ragers - shoot their heads (flak armor). Melee + warp slash and dancing around them works too, though it's less effective.

3. Bulwarks naturally, as there's no effective way of dealing with them fast, except heavy weapons (Rumbler, Plasma gun) or Krak.

Use melee for trash mobs + its warp slash for bigger hordes or groups of man-sized elites. It's very effective and conserves ammo. Moreover, you make great use of Soulblaze skills and Greatsword blessings. Try to maintain high peril to take advantage of Warp Splitting.
 

For unshielded bosses/Monstrosities, use Scrier's Gaze and keep shooting their weakspots. I can destroy Beast of Nurgle on Auric in approx. 2. mags this way.

For shielded bosses, I use Scrier + melee heavy, which is'n exactly effective, then quickly switch to ranged and dump the mag in their weakspot when their shield fails.

Blitz - you can choose Brain Rupture or Smite. Personally, I literally never use Blitz, but Smite is the better way to go in case you need to stun lock groups of Bulwarks/Crushers.