Havoc 40 No Frills Meta Psyker
 Psyker Build - Warhammer 40k: Darktide 

Very basic bubble psyker which will be your workhorse Havoc 40 build.

Kinetic Flayer, despite what most people say, is very useful, as it instantly front-loads your damage every 12 seconds. This is extremely valuable for your action economy because it turns your flame staff into a revolver, which lets you pop a stray special quickly. It also lets you start the Perilous Combustion and Wildfire chain more quickly. Psyker is about getting through "stuff" (hordes, carapace walls, elites) quickly and as a one point wonder it's very good at speeding your entire gameplan up.

Generally you'll be on flame staff 85% of the time. Use primary fire to stagger, hold down the flame to kill hordes. 

Alternative weapons are an Agile Atrox Mk IV Tactical Axe (lowest damage mark for best attack speed + triple special stagger combo), Maccabian Mk IV Duelling Sword (same concept as knife, trading slight mobility for better poke stab) and Deimos Mk IV Blaze Force Sword (no deflector, use it as a duelling sword with double stabs and push attack into stab).

Random tips 

Post Depths patch, tap quelling is no longer viable. We take quell speed because without it, the game actually sucks.

You don't need to charge to full every time on staff (in fact, I rarely do this). 50-70% charge is fine and is both faster and easier for peril management.

Aim up to make your bubble pop up ahead of you. Do this to save your teammates from sniper shots, flamers, trappers and gunner fire. You should try to get it so that you're aiming your bubble and then sliding into it. 

Get used to dodging while charging and shooting your staff. Standing still is a bad idea.

On duelling sword

DS4 is the money weapon. Really good mobility, really good stagger (spam those lights and push attacks), insane specialist sniping (dodge slide while charging heavy into headshot kills basically everything), gets uncanny AND also has a riposte if you want to panic against bosses.  

On knife 

Switch to knife to sprint-heavy attack away from stuff (do this more than you think, and before you think you need to bail), apply brittleness to bosses and pick off stray enemies when you're already at high peril. 

Positioning and action economy is absolutely key and the better the psyker, the more your teammates will feel like you're everywhere all at once. 

Either knife is useful but I slightly prefer the mark with 2 heavy strikedowns, as it uses less stamina than the one that does H > push attack (repeating). Unlike most classes (Which, imo, prefer mk 6 and flesh tearer to deal with hordes), Psyker is very much a horde clearer. The Mk 3 knife deals with its main weakness, which is single target elimination.