Quad-Burst Sentry
 Psyker Build - Warhammer 40k: Darktide 

Sustained Ranged
Shoot Lasgun in four shot bursts to make the most of fire damage / maintain toughness / Perfect Timing / Malefic Momentum. This ammo efficient attack pattern keeps up the pressure on enemies while your brain burst mag dump attack is on cooldown.

If you're having trouble hitting elites / specialists in mixed hordes with brain burst, I recommend aiming above your target and then lowering your aim toward them. Since their hit boxes are always taller than the chaff that surrounds them, doing this helps to minimize locking on the wrong target. You can also jump at the same time to increase your odds of hitting the right target further, but it's a bit riskier since you can't dodge in midair.

One of the main downsides to brain bursts is it's low mobility, so being able to approach with it ready to go is quite useful. The trick I found to do this is pre-emptively charge it while continuously running and sliding around. This is done by charging brain burst without locking on a target and then repeatedly running and sliding (RMB -> W+SHIFT -> CTRL). It seems that charging brain burst reduces your run speed, but doesn't have the same effect on your running slide speed. Also if you've moved a bit too forward and want to back up, you can always dodge backwards into a slide to a similar effect 
(RMB -> S + SPACE -> CTRL).

Similarly you can employ run sliding with your Lasgun, where you fire off bursts between AND on every slide (LMB -> W+SHIFT -> CTRL -> LMB). In practice you'll be walking during the first burst and sliding in the second. This quickly builds up Mettle stacks, making it a much faster approach option. It's so fast you'll quickly close the gap and be in melee distance of the target, so it's a better choice against basic hordes / light gunners. More often than not, though, you'll be using this technique to position yourself in the best possible location for your next brain burst mag dump.

• RMB (Right Mouse Button) = Charge Brain Burst (Without Locking on a target)
• LMB (Left Mouse Button) = Burst Fire Lasgun
• W + SHIFT = Move Forward + Run
• S + SPACE = Dodge Backward
• CTRL = Crouch (Slide when running)
   

Burst Ranged
-Place shields in choke points to disrupt specialist advances / access cover.
-With the Kinetic Resonance and Empowered Psionics talents, shortly after placing a shield you can quickly mag dump brain bursts when dealing with heftier foes.
-Perfect Timing increases the damage of brain burst whenever we get a crit. When fighting monstrosities, get max stacks of Perfect Timing with the Lasgun before mag dumping brain bursts. If you're being pestered by chaff enemies, kill them with the Lasgun instead since that'll also give you stacks of Malefic Momentum for the upcoming brain bursts. You'll be able to fire off 5 quick brain bursts before the damage buff runs out if you activated Kinetic Resonance by placing one shield (5/6 total quick brain bursts get damage buffed). This isn't considering the effects of Empowered Psionics being active so you may even fire off more in that time.
-Funnily enough, since we can spawn two Telekine Shield's at the same time, we can also activate Kinetic Resonance twice and they actually stack. This causes us to be able to quickly fire out 7 damage buffed brain bursts (7/8 total quick brain bursts get damage buffed) before Perfect Timing runs out. Definitely not something you can count on all the time but being able to wipe out elite hordes / chunk a monstrosity down to half health in seconds is pretty useful lol.

Melee
Use shock maul's wide sweeping attacks to clear hordes and it's special stunner attack + two light attacks right after to maximize single target damage from all sources (This applies full stacks of Skullcrusher  and staggers the target for Opportunist to apply). This three hit combo is best used when the target is already on fire to increase the damage over time of the flames OR with a quick brain burst to chunk their health quickly. Another niche but good use of the shock maul is against bubble shield bosses (Karnak Twins / Scab Captain / Admonition Champion). A quirk of their bubble shields is that they minimize all incoming damage to a certain amount, so fast hitting melee weapons that can get a lot hits in quickly are very effective.  By spamming the special attack of the shock maul, which is also applying Skullcrusher to the boss, you'll very quickly shred the shield due to it's multi-hit attack profile.

Alternatives
You can switch out Psychic Leeching for something else, like that tantalizing +5% Ranged Damage node. My preference for this choice is because it encourages us to liberally fire off brain bursts into mixed hordes even with chaff nipping at our heels since we'll quickly recover toughness mid attack. And while it feels bad to accidentally brain burst some chaff during these encounters, we can at least take solace in the fact that we were supporting the team by doing so.

Edit: 
After the "Bound by Duty" skill tree rework, this build got a bit of a glow up. Swapped out Warp Rider with Vulnerable Minds, since we don't generate peril consistently enough while brain bursting. With the extra freedom of choice I was able to swap out Lightning Speed for Malefic Momentum, for that sweet sweet damage increase across Melee / Magic / Lasgun. We're also able to get Wildfire, to further chip away at the health of enemies. Combine all of these at full stacks of effect (including Perfect Timing and Empowered Psionics), we're able to more consistently one-shot enemies with brain burst, like the Dreg Rager (armored variant). Also the new node Just a Dream is absurdly useful in this build, as we won't be generating that much peril compared to a more traditional staff build. The only L I think this build took, was having a tougher time killing crushers but I think its less a build problem and more likely that Crushers got buffed.