[DotD Update] "You are Astra Militarum, Harry" or Infinite Broomstick 3.0 [Auric and beyond]
 Veteran Build - Warhammer 40k: Darktide 

Fully charging Lucius Mk IV Helbore Lasgun with 60% charge stat constantly gives us 4 stacks of Surgical. 5th is very close and we'll naturally get one cuz of human factors like switching targets and correcting aim, resulting in whopping 50% Crit Chance!

Add newly re-balanced Deadshot that consumes way less stamina (+25%), Vet base crit (+10%), Opening Salvo (+10%) and single +5% Crit Chance perk on Helbore itself and we're getting GUARANTEED CRITS ON EVERY SINGLE SHOT. Thanks to Shock Trooper there's no need to reload what so ever, basically making Opening Salvo to last forever and we don't even have to care about ammo in the slightest~

If you want to be more supportive, you can also swap your Withering Fire, which for me works like 3rd weapon Blessing Hot-Shot that I like, to Field Improvisation.

Catachan Mk III Combat Blade with Uncanny Strike + Mercy Killer are good combo with Serrated Blade, Desperado, Precision Strikes and Trench Fighter Drill talents.
Single Target combo - Weapon Special > LMB Followup.
WS procs Serrated Blade and applies Uncanny Strike stacks for your next attack, which is followup heavy stab.

Superiority Complex, Grenade Tinkerer and Bring it Down! are needed to achieve one-shot against Havoc 40 Crusher with Krak Grenade.
If this feels like an overkill, then change Kraks for Shredder Frag Grenade and Grenade Tinkerer for Out for Blood.
Or for the sweatiest out there -> Smoke Grenade and Demolition Stockpile for +15% Melee Damage node, leaving Grenade Tinkerer in place.