Uncanny Purgatus True Duo Build
 Psyker Build - Warhammer 40k: Darktide 

Build Idea

Uncanny strike stacks can apply to the Purgatus staff's soul blaze, which make it a meta pick. Without uncanny its pretty meh and inferior to the Nomanus surge staff because it can't apply to crushers, and your monster damage is sort of meh. Apply a short spray of soul blaze along with some left click spam from the Purgatus staff on the crushers, vent shriek on them for good measure if possible, then whip out your knife and get stacks of uncanny strike as quickly as possible. The crushers will get deleted. As far as bosses, get the boss to 15 stacks of soul blaze, then vent shriek to get them to 21 stacks doing 1000+ ticks of damage, especially once you get some rending. Left click on them now and then to maintain the 21 stacks and you will delete any monster quickly. 

Recommended Mods

Download the "Debuff Indicator" mod to track stacks of soul blaze if you are on PC, its mandatory if you really want to make sure you are doing damage with this build. I set it to show the debuffs only on Crushers, Maulers, Reapers, Bulwarks, and all the monsters. Everything else dies really quick regardless. 

Staff Build

Make sure you get a low leveled Purgatus staff, with high stats in everything except for Warp Resistance, otherwise the staff won't generate enough peril and you will get rekt. You generally want at least as low as 45% warp resistance as a baseline, if not lower, otherwise your peril generation will feel pretty bad.

Optimize Venting Shriek

You'll want to first apply soul blaze with the Purgatus staff then vent shriek with at least 83% peril, allowing you to add another flat 6 stacks on top of the existing soul blaze stacks. Soul blaze does damage exponentially the more stacks you have, so those 6 stacks can add insane amounts of damage. 

Warp Siphon vs Disrupt Destiny

Warp Siphon is chosen over Disrupt Destiny for ability spam potential, allowing you to stack tons of soul blaze from venting shriek. True Aim on the way to Disrupt Destiny will also hardly ever be proc'ing without assail and using the Purgatus staff since you'll barely get any weakspot hits, so its just a waste of points. 

Brain Burst vs Assail

Brain burst is chosen over assail since the Purgatus already does good crowd control and can crit on its own (I usually get 1k-2k crits a match in pubs), allowing you to proc Mettle and Perfect Timing nicely without assail. Psykinetic's Aura is also insanely good and enables you to do even more ability spam.

Wildfire vs Kinetic Flayer vs Warp Expenditure 

Optionally, you can switch out Wildfire for Kinetic Flayer (preferred), toughness, or Warp Expenditure. Warp Expenditure is hardly giving any toughness though compared to your other talents like Mettle, Soulstealer and Quietude, so its really not recommended, but if you are a toughness maxer feel free to go for it.

Warp Battery vs Crit Node

That 5% crit chance node can feel pretty good, although feel free to switch it out for Warp Battery. Warp Battery is not chosen by default because most of the time you aren't actually reaching 6 stacks so its not always getting value, versus a +5% crit chance that is proc'ing all our crit talents that much more.

Combat Knife vs Deimos Mk IV

Take the Deimos if you want more potential damage, or the combat knife if you want more mobility and faster uncanny strikes. With the Deimos you can stack Slaughterer or Superiority - which both apply to soul blaze (thanks to both blessings being global power) - on top of Uncanny Strike. With this, you will be absolutely demolishing enemies if you stack both, though you may die more since you can't move as fast. The MK VI combat knife's block attack comes out a lot faster than the Deimos's attacks though, and since the knife can have uncanny, you will get stacks a lot faster and more consistent with it.