The plan that Lucifer told Luna was to use the citizens as bait to distract the vampire. With that, Lucifer would use Shadow Movement to get behind the vampire while he wasn't paying attention.
While the vampire was still focused on killing the citizens rushing towards him, Lucifer would suddenly appear from behind him and put the cross necklace around his neck.
That way, at least his power would be reduced. As the author, he believed that would happen.
Once he was reduced in power, Luna would fight him and defeat him.
This plan couldn't be carried out alone and required sacrifices because the vampire would definitely notice Lucifer approaching even if he was using Shadow Movement.
That's why Lucifer had to trade the lives of hundreds of people just to give Luna a chance to kill that vampire.
But there was one thing that Lucifer didn't tell Luna, and that was that there was still a chance that he would be discovered by the vampire when he went behind him.
