No Keystone Lastorrent Vet
 Veteran Build - Warhammer 40k: Darktide 

Nothing wholly original here.  It's a bit of a mashup between Mister E's MVP Vet and Crukih's Shadowstrike No Keystone Veteran that I play regularly on Auric Damnations and Havoc.  It melts down specials, elites, and monstrosities with a near endless supply of lasers and it comfortably chunks hordes with the chainsword.

How to:  Dodge, duck, dip, dive, and dodge to build up your crit chance using Reciprocity then shoot specials, elites, and monstrosities in the head.  Crits equal free ammo due to Shock Trooper and downing Elites and Specials nets you more ammo through Survivalist .  You get pretty consistent frag refills from Demolition Team / Demolition Stockpile , so use them to soften up armored mobs, monstrosities, and to help clear hordes.  After focusing down the high priority target, the chainsword will help you clean up the hordes.  You can also spray and pray hordes at head level to thin them out if you don't have the time to swap weapons - you have enough ammo to spare.  If you stick within range of your team, you'll be constantly topping up both your and their toughness thanks to Confirmed Kill and dropping elites.

Blitz: Shredder Frag Grenade  fit better in this build than Krak Grenade or Smoke Grenade .  Crushers and Bulwarks are no trouble for the stacks of damage, fire, and brittleness quickly applied by the lasgun, especially if you have time to lead with a frag to add bleed.

Ability: Voice of Command is the best choice here.  It's CC, toughness overheal, bounces dogs and bursters, clears space, interrupts mutant and monstrosity charges, etc.

Keystone: None of them fit my playstyle with this build, so I got rid of them. 

Marksman's Focus - Reload speed isn't a problem with our lasguns, crits don't spend ammo, and we've got a huge magazine. Added finesse would be nice, but I'm too old to have to think about juggling stacks. 

Focus Target! - There are so many elites and specials in Auric/Havoc that you're not going to get a full 5 or 8 stack frequently enough for it to pay off.  You could save up your stacks, but tagging my targets helps me pick out heads in the crowd, so I'd rather not have to save them for priority targets.

Weapons Specialist - I don't find myself swapping weapons enough here to warrant the payoffs.  I stick with my gun until the priority targets are downed, then switch to melee to mop up the chaff

Ranged Weapon: Accatran Mk XII Recon Lasgun is my go-to for this build.  It has the highest capacity and rate of fire, is fairly accurate to hip-fire at close/medium range targets, and pretty damn accurate in iron sights for long range targets.  Even if you don't land them all, you will be landing a ton of crits, so you can just keep firing.  You could make it work with either of the other marks as well. If you use the healthbar visibility mod, you can start to get a feel for how early you can stop shooting elites to let the burn damage kill them.  

Melee Weapon: Cadia Mk IV Assault Chainsword fits the need here.  Since I use the lasgun for all priority targets, I needed a melee weapon to clear hordes, defend and make space around me.  Knives and dueling swords were okay, power sword and shovels were better, but chainswords mulch hordes with lots of cleave.

Curios: I use a pretty standard 2x Toughness and 1x Stamina curio set with Ability Regen, Stamina Regen, and more Toughness.  The Stamina one could probably be replaced by something else like more Toughness/Health/Wound if you really need them - I don't often run low on Stamina as is, unless I get pinned into blocking a monstrosity or something.