Surge + BB for T5 above (+extensive description)
 Psyker Build - Warhammer 40k: Darktide 

A allrounder build that has become one of my favourite for running Auric missions. 

Offense:

This build focuses on using your Surge staff, you will be using it 70% of the time. Try to keep up above 50% peril for all those sweet buffs you get from talents. Your focus will be Elites and Specialists, most of the specials you can oneshot will full charged surge staff, which you will be spamming quickly thanks to the Warp flurry blessing. Use it for all close and mid range enemies, for those farther up use BB. When forced to melee whip up your duelling sword that can kill mutants on two heavy headshot attacks, and can stagger ragers out of their swing animation with alt attack. Always try to go for headshots to get the most from this high finesse sword and rending blessing. When you are facing a horde, try to get few charged staff shots into it and then use Shriek, when you are above at least 70% peril, it will knock most of the chaff down so you can shoot or BB mixed Elites and Specials while burning the chaff with soulblaze. For bosses use your BB - first use any EmpPsionics stacks and then use ult for another charge speed buff and continue, but you mostly want to be on lookout for any incoming specials. 

Defensive

You will have a lot of % toughness regen from talents. Your staff will crit a LOT, so 5% from every crit regens a lot over a run, I often end up game with 1k+ crits, so do the math. When in combat and low on toughness, you can quell for a second or two, if you have space or are quick with dodges, and the Quietitude talent will regen up to 50% of toughness on near full peril quelling (it's also why you should be on high peril most of the time). With how much peril you can generate with surge staff spam, learn the rhytm of couple shots and short quell, especially when facing a lot of gunners in open space, which is the situation you want to find yourself in the least. In terms of melee you want to focus on your sword excellent mobility which combined with Kinetic Deflection talent allows you to stay alive in chaotic melee encounter on par even with more defensively oriented classes. The block efficiency on curios helps you when you need to get that revive and Shriek for cc isn't up, when you end up against wall with boss because of bad positioning, or helps survive that one overhead attack from elite when you just can't dodge and have to take it. 

When to use BB?

Mostly use it on specials and elites out of range of your staff, or when you are full on EP stacks and there is a lot of elites like Reapers and you cant reliably target them with staff.

Isn't Warp Siphon better then EP?

Warp Siphon is an excellent keystone and you can't go wrong with it. Run it when you feel you have problem staying alive. Both peril reduction and and toughness regen talents are great, run peril when you want more staff spam and toughness if you are getting hit more often. In next choice take "In fire reborn" since you will be using your ult applying soulblaze on most hordes and with Wildfire it replenishes your warp charges quickly. To get talents for this build take talents from EP up to Warp rider (keep that) and apply them to WS tree.

So why are you using EP then?

I am quite used to my build and peril generation (so i dont blow myself very often so no ult cd is needed ) and managed to finally train myself to the point, where I am not taking lot of damage in my runs. So I am interested in doing more damage and removing threats quicker, since quicker special mobs removal = less chance of them fu**ing over you or your teammates. With Empowered Psionics my BB charges 50% quicker so I can get that one sniper screwing us over even in middle of horde. With the guaranteed stack on elite kill talent I can dispatch groups of elites much faster than I would with Warp Siphon. Situations I struggle a lot with are opens spaces with lot of elite gunners spread all around the map showering you anytime you peek out of cover. With EP I can just peek to lock them for BB, and with every oneshot kill giving me another stack I can demolish a significant group of elites in a VERY short time. So basically if you want quicker kill times take EP and for more defense and ult uptime take WS.

Additional curio changes:

The curios I have taken are made for an allrounder safe build. If you dont go down that often, change the +1 wound for toughness for even more survivability. Same with +3 stamina, I prefer to have it for situations where I get cornered but if you have premade group with dedicated frontliners and dont melee or clutch much, change it for +toughness too. 

In the end dont forget the basics of both Darktide and Vermintide - you can run even with most unoptimal build and still win if you keep your wits with about you, know your class and dont do rash decisions, so feel free to run higher difficulties even if you dont have perfect stats on your gear. Hell, my surge staff is only 351 rating with T1 and 2 blessings (curse you Hadron!) and I still often end up with most damage done and specials/elites killed by a ton at end of most T5 Auric runs.