From what I understand if the bow is just one of those slayers either Daemon or Elemental it will do double damage but if it is one of the old double slayers that has Daemon and an elemental slayer property it will do 4x the damage because of the way things roll. One slayer type takes effect and deals damage and the other slayer takes effect and doubles that damage. But those bows are really rare. I own one on Sonoma and I think I paid like 15 mill for it a few years back.