Parash - Havoc 40 Plasma Support
 Veteran Build - Warhammer 40k: Darktide 

Very conventional and very good, provides incredibly useful auras, points at things, prone to exploding if misplaced.

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Plasma Gun

Calculating breakpoints with Focus Target! and Skirmisher and the two plasma blessings and Longshot is a bit of a PITA. For subsequent notes, assume Havoc 40 enemy health, max Redirect Fire! and Skirmisher stacks (which are easy to max out during missions) and the minimal Longshot bonus.

• +Unyielding seems to be a gimme. 
  • With the perk, need only 55%+ plasma heat and 3+ FT stacks for an uncharged crit weakspot to one-shot a Reaper
  • Without the perk, you need all 6 FT stacks and be at 90% plasma heat for an uncharged crit weakspot to one-shot a Reaper
• Candidates for second perk:
  • +Flak is the most reliable choice - with it, you can uncharged non-crit weakspot kill scab gunner with 4+ FT stacks and 55%+ plasma heat. Without the perk you need more stacks of everything if you don't want to rely on a crit.
  • +Crit Chance is a bit of a gamble, but crits will often ensure one-shots on elites and makes bodyshots less bad. Gets Hot! means you'll be critting pretty often and they'll be disproportionately helpful. The bonus crit chance is always helpful, and it can make you slightly less dependent on heat gain for hitting breakpoints.
  • +Elites is interesting. Without +Unyielding you can uncharged crit oneshot reapers with 5FT stacks and 70%+ plasma heat. Without +Flak you can uncharged non-crit weakspot one-shot a scab gunner with 70%+ heat and 4+ FT stacks, or 55% heat and 5+ FT stacks. If you're feeling funny, can try +Elites AND +Crit chance.
  • +Maniac maaaybe? Lets you uncharged non-crit weakspot kill scab flamers with minimal FT stacks
• Can dump Thermal Resistance in exchange for Charge Rate so you can scale the plasma blessings faster. Been playing with that variation and it doesn't seem much less controllable. (Note that the optimal stats for maximizing heat gain is is 68-69% thermal resist / 71-72% charge rate, but the difference is pretty marginal).

Melee Weapon

• Shred or Riposte on the dueling sword helps with hordeclear. You already deal crazy armor and boss damage even without Uncanny Strike, your biggest threat in melee is getting bodyblocked by chaff and crits can be incredibly useful for lowering ttk on little guys. After trying both I prefer Shred - it feels more consistent, and is easier to proc while fighting bosses to boot (love juicy head-pokes on captains).
• If my team lacks AoE CC and has lots of boss-killing power already, I also run this build with an Aridin Mk I Power Falchion . In exchange for some mobility and raw single target damage, it provides easy, powerful CC rotations. It's a flexible weapon but my preferred setup is below
  • Dump stat Heat Management. Heat buildup is pretty reasonable, especially with blessings, and since you'll likely end up switching  weapons frequently to kill pop-up specialists you'll always have some natural cooling-off time. Isolated trash mobs can also be dealt with without needing to activate. Extra run speed is always useful!
  • Take Cranial Grounding . Second blessing Energy Leakage (most consistent damage) , Heatsink (higher activation uptime), or Overload (faster cooldown from max, some flavorful emergency CC)
  • I like+Maniac - the faster I can kill dreg ragers and finish off muties, the better. I prefer +Carapace as the second perk for the capacity to quickly solo crushers and maulers, and to contribute to killing Twins. +Flak is an alternative for better performance against scab ragers and dreg bruiser hordes.

Talents

tl;dr Born Leader and Focused Fire are the flex nodes.

• Close Quarters Killzone may be more reliable than Reciprocity, up to personal preference. Extra crit chance synergizes well with dueling sword and plasma blessings, so in theory the usefulness ceiling is higher. FWIW my Reciprocity uptime usually hovers around 25-30% with an avg of ~2 stacks.
• Born Leader is very, very helpful, but is probably the least essential node as built. The extra coherency radius facilitates more flexible positioning while maintaining coherence (remember - it's a daisy chain, so when your radius expands the impact on your team is nonlinear). The regen + shout bonus helps keep ogryns' huge toughness pools topped up.
• Agile Engagement is my current favorite alternative pick - synergizes well with the rhythm of keeping dueling sword out most of the time for movement, swapping to plasma to kill priority targets, swapping back for defense or poking armor/muties etc. 
• Can also swap for Trench Fighter Drill or Exploit Weakness to further capitalize on Shred/Riposte.
• Grenadier is an option, but IMO situations where you absolutely need to throw more than two smokes out at the exact same time are rare, so improving capacity from 3-4 isn't critical, plus 30s smoke duration from Grenade Tinkerer is plenty of time to regen with Demolition Team. In most cases there isn't a huge benefit from throwing two smokes in close proximity to each other, so the 20% chance of double nades isn't super impactful. All that said - if you're feeling spicy and your team rarely holds in areas long enough to make use of full 30s smoke duration, you could potentially swap Grenade Tinkerer for Grenadier .
• Covering Fire seems funny. Needs testing, but if your comp is sufficiently ammo-efficient, tossing a plasma at your friends every once in a while may pay dividends.
• Focused Fire is what I'd consider the second least essential node as built. However, it's effectively a flat 5% damage bonus for the whole team when combined with Redirect Fire! (you'll have 90% uptime, all at 6 stacks in typical games), and an extra 10% for the whole team against priority targets like bosses. That's a lot of damage to leave on the table.