I said Hotshot.
 Arbites Build - Warhammer 40k: Darktide 

Be prepared to draw your revolver like it's 1838. Line up enemies for infinite cleave megacrits, especially from range as the shotpistol has much less damage fall off.

This build makes the shotpistol a disgusting long range horde cleaver. Crits with the pistol are guaranteed if shooting a horde, have infinite cleave, massive stagger, massive rending, and melt everything from across the map as the pistol has very little falloff. Unloading a magazine into a crowd then also gets you to at least 15 melee stacks if it was a good one and 30 if it was a great one, at which point you can start hacking away like a maniac. 

Maul: Bonus vs Carapace and Unyielding to onehit crushers and dps bosses. If you are stacked up (actively fighting) then one single overhead heavy into the weapon special (the stun move) will kill a crusher from full hp. You don't need melee stacks on terminus decree to do this. But it's much safer to bonk them a single time more in an actual game because you're probably missing a damage skill's stacks. Or nade them before you fight.

Shotpistol: Damage vs flak and crit damage to deal with maulers and gunners. Maulers get obliterated by this gun as long as you hit them in the CHEST!!! I just played a game with this build where I killed six maulers with 2 shots because they were both infinite crit cleaves.

Grenades can let you deal massive area damage without switching weapons and losing the bonuses from terminus decree. They are also taken because you have a small magazine and grenades can easily handle groups of gunners that would take you multiple magazines to root out. They also synergize very well with castigator stance, popping castigator can allow you to stop hitting and pull out 5 grenades to vomit everywhere and make the room safe for a revive. Remember to use these proactively as they regenerate.

Relentless Strikes on the shock maul lets you burn down bosses in melee while castigator is active (or not). The shotpistol's boss damage is great with this build but you'll be reloading a lot so your dps will probably be much lower than a bolter's or a laspistol. Don't get it wrong though, if you unload a magazine into a weakspot while your build is stacked up the boss will be chunked HEAVILY. Have 30 ranged stacks and unload a magazine into the bosses weakspot then go into melee for the rest. You will do *very considerable damage, but nothing insane or game breaking.