Max damage waves (Warpshock Slash)
 Psyker Build - Warhammer 40k: Darktide 

Unstable Power Perfect Timing Malefic Momentum Kinetic Presence Warp Rider Scrier's Gaze Disrupt Destiny   

All of these increase the damage of the Force Greatsword's special attack, Warpshock Slash, enough to nearly or even outright kill Scab Maulers when fully stacked, not to mention any lesser enemies which should die even after damage falloff kicks in.

Usually, Crits on the Warpshock Slash will do only 30% more damage than a normal hit, but with this build crits are boosted by 60% through Disrupt Destiny and Precognition. So Warp Slice is a guaranteed damage boost for Warpshock Slash, between 30% and 48% more than without it.

And while Warpshock Slash cleaves a lot by default it can still be stopped by too much mass just like any other melee attack.
Unstable Power increases not only the damage but also the base Cleave potential, and it's multiplicative with Warp Splitting, ensuring that your maxed out Warpshock Slash can hit any number of targets that the game can naturally spawn at least. Without these Cleave boosts, I've had some Slashes stop before even hitting through a basic Poxwalker horde wave, just because it had some elites mixed in.

Most importantly, for this build, quelling Peril is a bad thing. 
You want to be near critical Peril (>97%) as much as possible, to maintain full power and stun immunity. 
Since that is the case here, these talents have no use for us: Quietude, Battle Meditation, By Crack of Bone, Just a Dream, Tranquility Through Slaughter.
Just so it is known that, whatever you change, those talents would only make the build worse.

Similarly, after testing Perilous Combustion, even on a Greatsword with Blazing Spirit, the burn effect was less than 2% of my damage on average.

Probably Empathic Evasion could be traded for Warp Speed or Surety of Arms though.

The Accatran MG Mk II Heavy Laspistol boasts the highest boss DPS that Psyker has acces to, because of its very high finesse modifiers. 
Great against specials too since it can by default cleave 1 or 2 horde enemies and stagger trappers and dogs into a stunlock until death.

If ammo efficiency is a concern, the Accatran Mk VIc Recon Lasgun can be used instead with the same blessings just switch the Perk from Maniac damage to Flak damage. However it has less DPS against bosses, also it's not as good against specials, since unlike the Laspistol  it does not naturally cleave chaff and it can't stagger them either. The usual meta pick on Veteran, Accatran Mk XII Recon Lasgun, actually has less DPS than the VIc on Psyker, probably because of the brittlenes being traded for finesse scaling instead, and the VIc does much better with higher finesse.

You could also use the Nomanus Mk VI Electrokinetic Force Staff instead with Surge and max crit, if you actually need infinite ammo. 
Might want to switch to Seer's Presence aura then so you have less time where you could blow yourself up.