>kill ghosts until Lucifer shows up
>????
>Lucifer dies
>game over
That's it?
I remember having trouble with this level before, and I don't remember why. But going into it this time, I knew there was some kind of gimmick, so I just tried attacking everything I could see except Lucifer himself (namely his sword and some boulders falling from the sky).
I just checked a walkthrough and apparently you're supposed to shoot the boulders into him and then shoot his sword back into him when he throws that. I wasn't even aware of what I was doing though. He was dead in one attack cycle.