Whirrun of Bligh's pyromaniac Havoc 40 build
 Psyker Build - Warhammer 40k: Darktide 

This is a great build for high level havoc for its supportive role, all-round good damage, and great horde clear potential.

Inferno staff is going to be your primary weapon used most of the time- you want to be utilising the charge up right-mouse attack as much as possible, charging it as high as you can every time while dodging back from enemies, so that you can make the most of the Blaze Away blessing. This means you will be taking a semi-frontline position (depending on the circumstance). You should primarily focus on horde clear as your top priority so that the rest of the team can focus specials, ranged, and bosses. This build makes quick work of hordes.

Use light ranged attack (LMB) for staggering enemies as they are charging, particularly ragers- it will keep you alive and give you breathing room so you can change to melee and kill them quick. This light attack is also great for pushing poxbursters back. However, you don't want to be using the light attack for much else- if you want to deal damage, use heavy charge atack (charged up as much as possible each time).

With Warp Siphon and combat ability regen on your curios, bubble will be coming off cool down constantly- pop it all the time to help keep the team safe from ranged, fire, and Burgles. Popping bubbles in between where the team is and where the ranged is tends to be the best way to utilise bubbles, so that the team has cover but also has a place to push forward if they need the toughness regen (unless of course you are trapped somewhere in a tight squeeze and need the extra help on the team in that moment, then it's always good to pop it over the team).

For melee, I go with tac axe because it's versatile, reliable, and great for staggering things like ragers. As soon as you get enemies within melee range, it is usually worth it to switch to melee to keep yourself alive, especially with the Agile blessing- lots of dodging, lots of push attacks, and making the most of the special attack into light attack combo. Heavy attacks are also great for killing specials and elites quick- aim for the weakspots! With bosses and monstrosities, you can stack some Soulblaze first with your staff and then I usually switch to melee for the blocks, dodges, and heavy attack damage.

You won't need to use Brain Burst much- it is mostly useful for the Kinetic Flayer modifier. However, particularly during ranged seeds, you may need to help the team take out ranged, so that's usually the best time to use it.

Enjoy ;)