Disabler Disabler
 Psyker Build - Warhammer 40k: Darktide 

Adrenaline junkies rejoice! Because now you can experience being a neurotic parent rushing back in forth between jobs while protecting your blunts and siblings. This is an incredibly versatile build that meshes with several different playstyles. The trauma staff's stagger is insanely powerful, at full charge it can trip and animation cancel disablers: Maulers, ragers, dreg gunners, crushers, bombers, flamers, trappers, and snipers. It can instakill bursters and even knock dogs off of your brutes. If kept at a high enough rate you can stunlock an enemy by just doing small charge spams.

Muties cannot be stunned, I recommend dodging and block→heavy (hold right click then left click) and then wailing on them. 

Reapers and bullwarks are the only enemies that will either go into a full knockdown animation or will be slightly staggered. For these enemies I usually pick and mix brainburst to kill them. If you're lucky and they get knocked down you can strike them with the knife and get a lucky crit. 

Team Role: Pick and mix to take out gunners and elite ranged, it is also viable to brain burst disablers that you can see far off. Once your warp charges are at 6 it'll likely stay up there without the need for brainbursting much. Inner tranquility's peril resistance allows you to tap into warpflurry better, you can get a lot of full charges off coupled with Psyker's Battle Meditation (10% quell peril on kill). You can horde clear amazingly with this weapon but to do so you have to angle the horde so that they don't get flung everywhere. I usually try to not hit them dead-center of the circle but to angle them away from the center point. Practice in the meat-grinder to see where you can re-position enemies. Sometimes flinging enemies forward such as a staggered hound or a flamer can allow your teammates to hit them while they're tripped.

The biggest problem with trauma staff is it's annoying crowd fling, try aiming for the back of the horde to thin them out if your melee teammates are trying to contain them. If you end up flinging an enemy and not killing them clean up yer mess! Use the knife to kill any dregs left groveling on the floor. Doorways are your friends, if you wedge yourself in one and aim the trauma staff at your feet you can really lock down an area. Usually if someone has a job, leave them to it, fill the job that isn't being done. Ogryn's got crowd clear? Switch to brainbursting elites or the objective. Flamer/Soldier needs to focus on elites? Switch to crowd control by aiming the circle at their feat or in front of them. This build really synergizes with flamers purgatus and zap staff by breaking up the bulkier targets and preventing your teammates from getting hit.

You're going to want to stay more near the back end of the team taking care of back spawns and disablers, you can trip flamers, trappers, and bursters no sweat. Follow up with a brainburst or knife hit to finish them off. Psyker with the knife-slide attack can allow for quick repositioning to the front or backlines or wherever you're needed. 

I've had a lot of fun running this build on Hi-Intensity Damnation. It turns the sluggish slow moving psyker into more of a zealot. Since psyker's toughness isn't great our biggest weakness is going to be regular dregs. Gunners and melee can pretty much shred you without a team to back you up. Retreat behind cover and brain burst to regain toughness against gunners. Keep melee off you by blocking with Kinetic Deflection. With this feat you can also slide in to do risky teammate saves so long as all gunners are taken care of. All else fails you can retreat back behind doorways to funnel crowds or aim the trauma staff directly at your feet for self protection, just mind the peril. I usually go with 70% as a safe full charge limit for trauma.

For bosses you can pretty much kill a plague ogryn with just the knife. Beast of Nurgle isn't as much fun because one vomit can reduce your wounds down to 1. I usually trauma staff if I have 6 full warp charges on the back, or pick and mix brain burst them while going behind to lead them away from my team and expose their back. Grimiores are also hell… but they're a fun challenge! 

I prefer an 80% damage traumastaff with rending because you can one-shot shotgunners. There is however a really cool build showcased by j_sat where they show off how great warp flurry and transfer peril can be for this staff: https://youtu.be/wfmj9qFhnkg