Zapper with gun
 Psyker Build - Warhammer 40k: Darktide 

You use a gun because you want to play the typical Scrier's Gaze gun Psyker.

I use a gun because a staff sucks at killing bosses.

We're not the same.

1. Kill some Elites
2. Cast Enhanced Smite until critical Peril
3. Cast Venting Shriek without releasing your Smite, until critical Peril again
4. Cast bullets
5. ???
6. Profit

For specialists, shoot them until they're dead.

For elites, shoot them until dead if your shriek didn't get them. Or stab their heads.

For bosses, shoot them from flank/behind until they're dead.

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Addendum:

1. Enemies will not stagger if you reach 100% and enter auto quelling with Smite. Stop before that and Quell so you have enough time to do a second Smite or just switch to your weapons.
2. Try to keep your Peril at higher numbers during combat, except when casting Smite. You get damage bonus and Toughness reduction through high Peril, and Smite can only overload when you cast a LMB at critical threshold. You also have Venting Shriek as panic button. Be bold and run your Peril high.
3. There is another way of keeping Smite on with Scrier's Gaze/Warp Unbound. It gives you the advantage of a Gaze Gunker but you don't get the utility of Venting Shriek. Up to you.