Canary's 'Havoc Plasma Gun' Focus Shout
 Veteran Build - Warhammer 40k: Darktide 

The build I mainly run in Havoc, but also just in general if I ever want 'a supremely effective Plasma Gun build'. Gets basically everything, max toughness (run around with 200 at havoc 40 which is kinda insane, even if you'll still get 1 shot by an overhead), One motion + Tactical reload for the Plasma Gun, survivalist so you can actually shoot that Plasma Gun, + Fully Loaded so you'll get 2 shots instead of 1 per proc of it. You even get Superiority Complex, to help with all that extra armor and durability on those higher difficulties. 

About the only issue is your spongy nature, but that's what the golden toughness from you and your team is for. And the fact you don't get Field Improvisation, but one can always drop damage for that if it's really needed. Beyond that, you're pretty stacked with basically every good thing under the sun, even have increased move speed so when you need to swap to your Duelling sword you can dance around relatively safely. Focus Target for more damage gains larger foes, Shout for as stated golden armor and spacing along with Frag grenades.

You are just the quint essential Veteran, ready to f$%k, sh!t, UP! (just don't get bonked, and you'll be golden, literally).