if I got -10% target elemental resistance reduction on an Enemy and then i put -10% target fire resistance reduction on them too. Will they have -10% or -20%? I know they don’t stack but I’m not sure if just fire doesn’t stack with fire or just all “target XY resistance reductions”
-% does actually stack, so in your case it should be -20% fire, -10% cold and -10% lightning resistance. The two other ways of reducing resistance do not stack.
What I don’t remember is does -% resistance stack when it comes from two identical sources, probably not.