I didn't look into this really deeply but the old elemental wheel has changed at some point, apparently making mobs astral or umbral. This would need to be checked better, but it seemed to work for most mobs I have encountered.
Most of them (or all?) are weak to a certain element, get "normal damage" from 2 other elements, and reduced damage from 3 others. An Ixal for example is wind and thus an "astral" mob, he's weak to ice, gets normal damage from earth and water, and gets reduced damage from lightning, fire and wind.
Still, where is the water spell?!