Firestorm Psyker
 Psyker Build - Warhammer 40k: Darktide 

Spam your Nomanus Mk VI Electrokinetic Force Staff's lighting at all times, except when there is a distant sniper/gunner no one can reach.  In that case Brain Rupture them.  Venting Shriek on the largest group of bad people you can find when your peril is 83%+.  Venting Shriek penetrates cover, walls and terrain so enemies don't even need to be in LOS.  You can prime your Venting Shriek to be even more effective if you haven't reached 100% peril:  Swap to your Covenant Mk VI Blaze Force Greatsword, do a quick Warp Slice with your weapon special, then Venting Shriek.  The Warp Slice will be a guaranteed critical which will prime the enemies with 4 stacks of Soul Blaze.  The below video demonstrates this during the end event at the tree.

The Nomanus Mk VI Electrokinetic Force Staff's lightning can shove bursters away safely before detonating, just watch out for allies.  The Covenant Mk VI Blaze Force Greatsword only exists for the Warp Slice every 15s and its defensive ability to get you out of a jam.  Don't bother hitting enemies in melee.  You should be using lightning at all times to power up your Venting Shriek with peril.

Step 1: Open the fight with a warp slice on the largest group of enemies.

Step 2: Spam lighting until peril maxes.

Step 3: Aim for the largest bulk of enemies and scream.

Stay at high peril to maintain your buffs from Warp Nexus, One with the Warp, and Warp Rider.  If you need to manually quell while Venting Shriek is on cooldown just hit the quell for 1 second then go back to blasting lightning.