Zapinator 9000 - Highest Crusher DPS in the Game
 Skitarii Build - Warhammer 40k: Darktide 

This is the highest Elite/Crusher damage build in the game currently. Data Tide analysis in this post: forums.fatsharkgames.com/t/more-people-need-to-try-arc-rifle-advanced-combat-doctrins/123274?u=jdls

Please note this is not META this is max damage while still being viable. There are many tweaks you could make to make this build more well rounded and consistent. I will post a META version eventually, for now there is more detail on how to make this build more consistent and safe lower in this guide.

Overall Concept

This build ignores almost all survivability for the sake of a fairly consistent +150% damage and +30% fire rate/crit chance. This turns the Branx Mk IV Arc Rifle into one of if not the most powerful ranged weapons in the game, combined with incredibly good ammo efficiency and fairly decent ammo generation this build WILL top damage auric maelstrom or at least be a strong contender without picking up much if any ammo. This is even a viable build for no ammo pickup missions so long as you are more conservative with your shooting and only use it to clear large elite hoards. I have not tested it in Havoc however the ammo efficiency and the fact this build's damage scales very linearly with enemy density, makes me think it will be very capable in havoc.

Playstyle

This will primarily play like a weapons specialist veteran. Using the heavy attack pattern of the Branx Mk XI Paired Transonic Blades to get as many headshot kills as possible off light hoards for Reactor Coil Recharge (I recommend setting your height to minimum to make headshots more consistent). This is enough to sustain Advanced Combat Doctrines as you only need it for brief bursts most of the time, however if you find yourself requiring more capacitance dropping Weakness Analysis Doctrine or Servo-Core Recharge Engine for Flux Conduit Build-Up should be enough of a boost. 

Whenever you see a large group of elites or heavies activate Advanced Combat Doctrines and simply hold M1 until there is no longer a large group of elites or heavies. (Shooting Bulwarks in the exposed arm is also viable and should allow the arcs to bounce off of them and hit other Bulwarks in the back/sides). Dumping a entire magazine into a large hoard containing elites is the optimal damage output and the ammo efficiency is enough to sustain it comfortably, you should be mindful about how much you are shooting but not worry about it, I can shoot almost a entire game only picking up a handful of small ammo pickups or a single large ammo bag. I at times will dump two magazines or even more into particularly large hoards, the longer you shoot the better the Branx Mk IV Arc Rifle becomes as more enemies become electrified Calculated Priority stacks and Writ of Ammunition Enumeration reaches +30% fire rate that is a massive boost to Branx Mk IV Arc Rifle damage due to it spreading electrified faster. (Advanced Combat Doctrines does not drain capacitance while reloading). Weakness Analysis Doctrine in my experience is enough to sustain Advanced Combat Doctrines for quite some time and even go slightly positive on capacitance. 
 

One of the more important things when it comes to ammo conservation, especially since you are not spamming your ability like with Chordclaw Strike. Is to use and abuse Noospheric Command at almost all times. It is a incredibly useful tool to remove specialists, disablers and elites, it will even stagger gunners out of their shooting animation. It can easily be triggered while in melee (I recommend making a macro to double tag or simply using single tag companion just be mindful about tagging pox bursters), and in my experience will consistently kill 2-3 elites especially with Infernus stacks. This is Skitarii's version of Blades of Faith and it should be used in the exact same way, this will save you a large amount of ammo over the course of a game simply picking off stray enemies rather then using your Branx Mk IV Arc Rifle.

This is fundamentally a backline support build, you should be in melee but as soon as melee becomes dangerous you fall back and start shooting. You really shouldn't even be engaging more then one or two raggers without switching to your gun, and you should not be pushing into areas first. You are rather delicate but playing more as a DPS support like a Psyker will net you good results with the added bonus of being able to revive anyone who does go down easily with Medicae Servo-Skull. Do melee but prioritize shooting when the situation demands it.

Okay but what if you want to survive

This build can drop numerous talents to become more survivable and less of the (slight) meme that it is. Being a complete glass cannon is fun but not exactly super viable outside of certain team compositions. For my personal picks to make this more survivable:
 

Left of the tree: Drop Last Stand Relay and Channelled Motive Force . Take Residual Current Buffer and Omnissian Recharge Litany 

Top of the tree: Drop Weakness Analysis Doctrine and Retribution Conduit. Take Electro-Strike Conduit (Helps to give breathing room from spamming light attacks)

Right of the tree: Take Threat Detection Imperative 

Doing this should make the build reasonably survivable without sacrificing too much damage. This can be played with to your liking but I believe this could be a good starting point to make this less min maxed. 

Other choices to consider

Sustained Assault Doctrine: Good melee damage bonus that also can buff your shooting. Unlike Hybrid Combat Covenant this does not decay one at a time and for that reason I didn't put it on this version of the build, but it is still a solid choice to buff your first few shots

Resurgence: You don't really need the extra 10% ammo much and better coherency regen is very nice

How to max your DPS

This is very important for this build. You should when beginning your salvo rake your fire across the enemies. Arc has a chance to jump between enemies but it is not 100%, shooting all the enemies you want to kill once to 100% electrify them is optimal rather then focusing on one target at a time.

Once all the enemies you want dead are electrified ALWAYS shoot the enemy closest to you. This is extremely vital as the arc rifle arcs in a cone away from you on target hit. Shooting the closest target will maximize how many enemies it can jump to, resulting in a very significant damage boost.

This is absolutely vital to your damage, you need to actively avoid shooting one specific enemy and constantly switch to the most advantageous target to hit. In my testing this is the difference between killing 4 maulers and 4 crushers in ~16 seconds to killing that same group in 10 or even less (keep in mind these numbers are without all the buffs you will have in active gameplay as I tested without power overload and static capacitor drain active, and you are likely to have more calculated priority stacks when shooting into real hoards).

Maximizing Calculated Priority is also reasonably important. If you are trying to kill crushers and there are some ragers or shotgunners around, shoot them first, especially if they are in front of the crushers. That +25% damage is nothing to scoff at and is worth shooting less valuable targets to build up quickly. 

Calculated Priority also has another interaction where it lingers for 10 seconds. The fact that Advanced Combat Doctrines only uses 25% capacitance to activates means you can toggle it on just for a few seconds to shoot into group of gunners/shotgunners. This lets you build a +25% damage boost for a good bit without spending much capacitance, this is also a flat damage bonus not just ranged damage so consistently using Advanced Combat Doctrines while doing melee is rather powerful especially if you also choose to take Sustained Assault Doctrine instead of Weakness Analysis Doctrine.

Why no Purgator Servo-Skull 

While a good and rather high damage potential blitz, its not an amazing blitz and tends to fill the same role that activating Advanced Combat Doctrines does (shooting into large and tough hoards). I find that you kill enemies fast enough that it is unlikely you will actually get much benefit from Purgator Servo-Skull and simply saving the grenades for Medicae Servo-Skull is a much better use generally. Not taking Purgator Servo-Skull also ensures that you do not accidentally waste a grenade on it while trying to revive someone. Overall while Purgator Servo-Skull is good, I find myself rarely if ever using it and that last trait point is better spent on other things.

TLDR

Weapon's Specialist Veteran but with even more damage, useful servo skull revive, and a servo skull with a infantry lasgun that has infinite ammo.