Heckraiser isn’t supposed to be about killing the mobs, it’s looting a legendary from the event. I’ve gotten 5 today and no unlock yet. Unless I am misunderstanding the challenge. I’ve killed the end boss twice or three times now. Got credit for those. Got credit for the 50 weapons, for the ghosts I’ve killed, so far everything I know I’ve done except Heckraiser.
Edit: and I think I’m getting loot blind, and misunderstood your post. I’m going to step away from the controller for a while after editing this.
I’m not even sure that the version of the rare mob was even the RIGHT version of it, because I think (and could be wrong) that the haunted versions look different, and this one didn’t, even though a ghost came out of him, and I can’t promise I managed to kill the ghost though I think I did. But I might not have.
I have a ticket open with 2K on the Challenge not unlocking. I sent one of my legendaries to someone on my friends list and they don’t think Heckraiser unlocked for them either. 2K has a whole list of stuff I’m supposed to do like setting my console on fire and throwing it out a second story window, sending a cheek-swab and a blood sample - a whole list of stuff I need to do still yet (supposed to delete the game, delete my game saves, clear my cache and redownload everything), but I just hard restarted the console and I’ll see if that helps. I’m laying down for a while, too darn jittery from farming too hard and selling off 20 million in loot for the day so far.
God I love in game events.