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 

I find double-tap quell to be more useful than holding quell. You can generally sit in the 80-90% range of peril (going up to 99% as the situation demands) to safely doing your damage.

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.

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 prefer the mark with 2 heavy strikedowns, as it uses less stamina than the one that does H > push attack (repeating).