"Holy Water & Talisman" 

[In many novels, we can get rid of demons by using some chants or holy water. Maybe if the purifying ability users and the light element ability users infuse any water with their abilities, they will create holy water that can kill demons?] 

[This one sounds like a plausible idea. From what I know, purifying ability users are usually called priests and they also have their own guilds. Won't they also know how to make holy water?] 

[Our mercenary teams often go to temples or churches to find priests when we are entangled with some kind of strange, negative buff. They did sell holy water.] 

There were many things in this world that needed to be purified, for example, a curse,  or a wound left from a corrosive element, or wounds left from a dark element ability. 

People who could heal negative buff plus wounds or even regenerate people's bodies existed, but priests with their purifying ability also had many uses.