The immortal bar patrons in Night of the Comet. They're immortal. You can't kill them.
This is a known bug. It's been known since may- possibly even before.
But it hasn't been fixed- now, maybe it's not top priority to fix a bug that will absolutely ruin the game experience for new players by preventing them from completing a mission that is needed to unlock the next set of missions- and thus from advancing beyond that point.
But if it isn't- (as is obviously so), I think the question to ask is: Why?
Again, this is a known bug. There's no excuse for not fixing it, especially with the change to how missions operate. Before you could just never finish that mission and go on to greater and better things. Now you can't. Seems to me like the constant platitudes of "We have other things that need attention" need to go away, and you need to fix this bug.
EDIT: I've submitted a ticket # 17,021. I've also submitted other tickets before, but I don't remember their numbers- only that you refuse to fix this bug.