Post season 1 Treyarch reverted the boss fight in Cursed to it’s original harder difficulty.
To unlock Hard Mode dark ops challenge you need to Complete Ashes of the Damned Main quest with the Blood vials, Dragon, and Bus relics Active
Relics effect:
Bus - Enemies regenerate health (including the boss)
Blood vials - All augments are turned off
Dragon - Ammo crates are disabled