Ooga Booga One-shot Zealot
 Zealot Build - Warhammer 40k: Darktide 

Ooga booga, go one-shot bosses. 

At the start of the game, find a red barrel and drop your hp to one wound or two if you're a bit scared of dying.

For more horde clear and less tank, swap Shield of Contempt for Against the Odds in the middle of the tree.

To one-shot a boss: Use Shroudfield, activate special attack, heavy-attack-backstab-headshot the boss. 
Waiting too long or doing other actions will cause the first heavy attack to be from the side. To prevent this, immediately start charging the heavy once the special animation finishes.

Specific targeting to get the headshots:

Chaos Spawn - have to hit the head 'through' the back, stand a little off center to the right, aim somewhere between the vials on its back.
Plague Ogryn - from straight behind, aim at the vent on their back.
Pack Master - head is carapace, hit his right arm instead.
Beast of Nugle - just hit the pimple.
Demonhost - try to do it before she wakes up, but just hit the head.
Lieutenants/Twins - multiple headshots required because of a 20% cap on damage taken in one hit, but just hit the head.
Captains - when the shield is down, just hit the head