Slayer ar works on a chance to proc situation. 6 pieces = 100 chance to proc the slayer reduction. All pieces are weighted equally so each slayer item gives a 16.7% chance to proc.
A couple pieces can work to mitigate continuous damage but if the mob can hit you with surge damage (back to back fs) you run the chance of taking the full damage on both even with 5 pieces.
A couple pieces can work to mitigate continuous damage but if the mob can hit you with surge damage (back to back fs) you run the chance of taking the full damage on both even with 5 pieces.