What exactly do you mean by you can't kill him? Can you not get into the battle?
If you can get into the battle, I would suggest trying to find the best way to destroy the Pacifier and his friends. Maybe try your new upgraded rifle.
Well, the part of the game you are on is free so that shouldn't matter much.
Hope you get it. If you need more help let us know.
Isn't this topic like the one I made?