Fire & Bubble Witch - Psyker Meta Havoc 40
 Psyker Build - Warhammer 40k: Darktide 

The most important skill to learn is how to kite away from crushers and monstrosities. 

Try to place your bubble in smart places, either put it on your team or on shooters if you can or inbetween your team and shooters, try to use the maps' natural covers and chokepoints so the shield doesn't take excess damage leading it to failing shooner than it should although sometimes it can't be helped. 

Elites and specials are your friends, stick close to them. They're non ironically your lifeline when deep in the fight and one of your greatest sources of damage since they cause chain reactions with Perilous Combustion and Kinetic Flayer while your shield should keep you safe from flamers, snipers, trappers and shooters in general so you can stunlock everything with the staff's M1/Left click while your M2/right click suppresses the lesser horde enemies (beware the Contaminated Stimms modifier it can cause some enemies to ignore stagger/suppresion and catch you unaware)

Try to not get bosses attention until things are under control so you can also chip in with your knife or stack brittleness on them with the staff. 

That's it. That's all you need. 

Some people only take one between Wildfire and Empyric Shock, or/and don't take Empathic Evasion, saving up 1 or 2 points which can be spent on Vulnerable Minds, Anticipation, Warp Ghost, Just a Dream, Lightning Speed, Kinetic Resonance, Warp Expenditure or Soulstealer

The only weapon you need is your staff so your melee is completely optional but it's always good to pick something with good armor/boss damage or mobility if not both. The good ol' "what you're good at" applies here. 

The unchanged meta since Havoc 1 came out. Infinite waveclear plus the shield utility is not easily outclassed and the only lacking thing about it is the boss damage but using certain melee weapons can offset that such as any of the knife variants, the Maccabian Mk IV Duelling Sword, or any of the psyker-specific force swords with the exception of the Illisi Mk V Blaze Force Sword which is an ok weapon but not superb as every other of its family.

Tzeentch must really hate this build specifically because it never changed so far. And hope it never does, people sometimes hate on meta-slaving because it's so overused but there is something novel about walking around being able to stop bullets with your mind and conjuring a firestorm as if you were dumbledore. It's the essence of psyker gameplay fighting heretics, demons and hopefully soon aliens, while also fighting your peril almost exploding from the effort but hanging on by a thread that sets it apart from the other classes and other games in general.