Havoc 40 knife Zealot martyrdom
 Zealot Build - Warhammer 40k: Darktide 

Knife: Uncanny Strike is still very good even after the nerfs. Riposte and Flesh Tearer can also be good if you don't want to run uncanny strike.

Flamer: use to clear hordes and apply brittleness to large groups of armoured enemies, throw knife when swapping to flamer to cancel animation.

Curios: 3 wounds or 2 wounds+1 toughness, 3x revive speed is extremely strong.

Your goal is to stay alive, chorus guarantees revives + you have crazy revive speed so the best tip is to play defensively, it's not your job to deal damage.

Fully channel (7 pulses) your chorus for toughness + dmg boost / you can vault over objects while channeling chorus or move faster by dashing sideways / try not to overlap with Voice of Command yellow toughness if possible.

Use chorus to help teammates grabbed by a mutant, they can deal crazy damage on high havoc.

Use med station if you have 5-6 corrupted wounds, otherwise leave it for teammates.

Save your flamer ammo as much as possible, it's incredibly strong for dealing with hordes. If you have a vet with ammo regen in your team you can abuse the flamer as much as possible, it's just insanely strong. You can also use the flamer to damage bosses if you don't want to risk getting hit (especially against beast of nurgle).

For twins, use chorus when bubble breaks to stagger and kill the twin before next bubble, remember to use the push attack to get attack speed from Punish Impiety to destroy bubble faster.

For bosses, use chorus to push them into a corner or into your team so they're easier to hit, for beast of nurgle, chorus does nothing.

Gunners/reapers are by far your worst enemy, pray your teammates deal with them, you can also suppress them with flamer if needed or with chorus - if they have the pink havoc modifier they're basically immune to stagger and suppression so you can't do anything against them.

If you're running Blades of Faith it's often your job to kill specialists since you can oneshot them. They also deal good damage against unarmoured enemies like reapers/dreg ragers/mutants.