GS-Assail High DPS Auric Maelstrom viable
 Psyker Build - Warhammer 40k: Darktide 

Overview 

A very offense oriented build, where you basically deal with different kinds of enemy by attacking them with different ways, Tested repeatedly in Auric Maelstrom missions and almost always comes back with excellent results, even when playing cross-continent with 150+ ping. 

Explanation:

The build allows you to be effective against almost every enemy at almost every range in the game. 

• Assail is effective from mid to long range (especially if charged) against unarmored targets. This is your prime tool against Gunners/Shotgunners/Ragers (provided they are not in melee range) while the charged Assail is good vs snipers, or you can also use it to horde clear from a distance. 
   
• The Greatsword light attacks are great at managing hordes and with Scrier's Gaze + Warp Splitting, you can build up the full charge for the special attack within <6 seconds. The heavy offers decent AP damage but due to the weapon swing, its pretty easy to hit the heads of Crushers and Maulers. Combined with +25% dmg to Carapace and +10% Elite dmg + Scrier's Gaze and Warp Rider, you can now reliably deal with armored enemies in melee. 
   
• The Bolt Pistol's capability is basically as good as your aim; its accurate, it hits hard, it's relatively ammo efficient and it has a quick draw speed. It gives you now an option to deal with Carapace Armor from ranged, it helps you kill dangerous specials quickly (i.e. a Pox Bomber about to throw a grenade or a Flamer about to cook your Zealot) and with the blessings and talent from this build, Scier's Gaze magdump can deal a LOT of damage to bosses. 

How to use:

Aside from basic mechanics, there aren't a lot of complicated things in this build. Scrier's Gaze is a general buff ability, the Bolt Pistol isn't like a staff that has a charge mechanic and keep an eye on your peril when using your Warp attacks. Play intuitively, this build is designed with flexibility to deal with multiple in-game situations that you can execute with pretty basic mechanics. 

Scrier's Gaze and swing your sword into a horde, when the gauge is full, use the special attack to clear half the screen. 
If there is a Rager pack rushing you, or there's a Shotgun pack Scier's Gaze and Assail. 

If there is a Crusher/Mauler train, Scrier's Gaze and headshot with Bolt Pistol or heavy melee spam. 

If there are multiple gunners behind cover shooting at you, launch charged Assail to counter-snipe them. 

Block/Push Poxbursters, dodge Trapper nets, don't use Assail in melee, etc. 

There really isn't much to say, just do your thing and because of how flexible this build is, chances are you will find your fit with it.

Switching Talent/Equipment(s):

1. You can switch Empowered Psionics to Disrupt Destiny. Empowered Psionics gives your Assail a more significant buff to its "burst" damage potential; 100 to 150 doesn't seem like a lot but remember that they hit multiple targets. Disrupt Destiny on the other hand is better for general use; its buff is (very) easy to trigger with Assail and Greatsword and the bonuses are going to be helpful in most situations. 
    
2. Warp Speed talent is really not necessary, I just put it in because I like it but you can feel free to switch it to whichever you prefer. You can for example move the node below Prescience to the middle and then take One with the Warp. 
    
3. Before the Greatsword came out during the latest patch, I used Illisi for this build and it worked fine. I still prefer the GS because Illisi lacks the AP heavy attack, not to mention the screen clearing special attack. However, Illisi does have a remarkably faster wield and swing speed which gives you more margin of error when it comes to dealing with sudden (or bugged silent) Poxbursters for example.
    
4. I think everyone knows the state of the Dueling Sword now, it is ALWAYS an alternative and there's no need to go into details regarding that.
    
5. The Bolt Pistol is a bit more of an unorthodox choice given that most gun-Psykers use Recon Lasgun or Autoguns and you can well use either of those if you so choose. I personally choose the Bolt Pistol because it feels more reliable to me; it doesn't rely on crit proc (though I'll admit its not that hard to proc crit with automatic weapons) and it uses less ammo. For the tradeoff that I need to aim carefully to make my 8 round magazine count, I think its fair. 
   
   P.S. Also I have tested this Bolt Pistol build vs the more "traditional" gun-Psyker builds in Auric Maelstrom and honestly it comes out pretty neck to neck.