Fire and Ice wittch viable?

So, the Elemental Equilibrium passive looks like it might be neat to play with by mixing fire and ice spells, but I'm not sure exactly how it works; does it do anything to mobs with 0 resistance, and do most mobs have some resistance, or are they all 0 resistance if they don't have a specific buff?

Assuming it works against most enemies, does anyone have any suggestions for a fire and ice build that uses it?