something i'm noticing which i don't remember ever encountering outside the beta is enemies that are immune to all types of elemental damage (fire, ice, lightning, poison). if i hit them with a skill that only does elemental damage, such as fireball, lightning flash, ice trap, etc, they always take 0 damage. if i hit them with a skill that does some skill damage, such as dervish, they take damage roughly equivalent to the skill damage portion. in my case, that's around 100 damage. for comparison, hitting an enemy that's not immune does around 14k damage.

some examples of enemies i've seen be immune to all elements are ectoplasm, jiang-shi, jenglot. i've tried enemies in hell gates and in lower level areas and it seems to be the case everywhere for these enemy types.

bonus bug: gem effects socketed into an equipped item such as "+126 additional fire damage" don't affect skill tooltip damage or the character sheet, but item affixes like "+410 additional fire damage" do