Warhammer 40,000: Inquisitor - Martyr

Spec for health, equip any melee weapon, Sword, axe etc from your inventory and run missions until this pops, generally find that swarming enemy is the hardest to deal with as your health will get destroyed quickly. Best to run the random missions (LT) on the star map to select lower-level missions to grind out.

Through trial and error, I have found that you can also use the level the archeovault early in the game to farm this. Once you have gotten to boss don't destroy the Nurgle pustules and stand back but the doorway, the will come in waves and the boss won't come near or you can kill the boss and not complete the mission. But waves will spawn I am working in this, got the vermin achieve also farming the boss at the same time then quitting when he is dead and reloading the same level.

I was getting progress towards;
Angel of death
Relentless slaughter
And
Vermin slayer