Offense is Defense Flame Staff Psyker
 Psyker Build - Warhammer 40k: Darktide 

Suitable for Havoc 40

For your Flame staff, prioritize getting your Warp Resistance as low as possible, while hitting the listed numbers for Damage, Burn, and Cloud radius. Quell Speed can be dumped to a degree to get your Warp Resistance lower. My personal staff is 19% Warp Resistance, but lower is both possible and better. 

After getting enemies to max Soul Blaze stacks via your flame staff's alt fire, you have a few options: 

* Empyric Shock will increase your warp damage, including DoT. The most efficient way to do so is to sprint cancel by clicking primary fire, tapping sprint to cancel the animation, and then clicking primary fire again. This is also useful on it's own, and builds peril rapidly.
* Shrieking Vent and Perilous Combustion will allow you to add more stacks past the max of your staff, increasing the damage ticks considerably, as Soulblaze stacks are exponential in damage. Going for 10 to 11 is a bigger increase than going for 4 to 5, considerably.
* Pull out your knife to stack Uncanny Strikes and Executor. This is usually something you do on armor packs containing many crushers. This will see frequent use against the Rotten Armor modifier. 

For bosses, get them to full stacks, fully stack them with Empyric Shock, then use Venting Shriek. While they're burning, and debuffed to take 30% more warp damage, take advantage of Kinetic Resonance to get reasonable levels of boss damage in. Don't overinvest in doing this, as it is not your primary role. But with good positioning, you will be attacking horde at the same time you're building up Soulblaze and Empyric Shock on the boss, and Venting Shriek will CC enough to allow several uses of Brain Burst. 

Outside of building your DoT damage against Crushers, your dagger is mostly used as a mobility tool. You can dodge and sprint rapidly to position yourself to best do your job, burning enemies. Don't be afraid to kill lone enemies with it, but otherwise, flame and brain burst should be doing the vast majority of your damage. A couple of movetechs to note:

* While sprinting and holding heavy attack for sprint speed, if you tap your keybind to swap to your melee weapon at a reasonable pace, you will extend the held attack, allowing you to sprint farther. When doing this, you want to be using your second or third heavy attack. 
* All dodges have a maximum distance they will normally travel. This limit is largely ignored if you're not touching a surface. Therefore, you can dodge off of raised surfaces, such a stairs, to increase your mobility. 

Brain Burst is best used when Kinetic Resonance is active; if at all possible, delay for your Venting Shriek to be active before taking out units with it. This is doubly important because your Venting Shriek also CC enemies, buying you time to actually do so in the first place. 

The special attack of the Inferno Force Staff can stagger most enemies, including reliably knocking Crushers out of any attack barring their overhead. 

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If you choose to use mods, consider "Guarantee Ability Activation", "Guarantee Weapon Swap", and "Healthbars". Healthbars, in particular, is useful for the option to see Blaze Stacks active on enemies.