Some attacks from bosses appear to be elemental, but in actuality they are not. For example, Ephenia's poison mist deal DoT that is not poison magic because it is not cut by skill "Partial Resistance" of my F/P and deals solid 3000 DoT. I don't think the skill is bugged, because it properly halves Papulatus' fire attacks and not ice or lightning attacks. This also seems to be the case of some elemental attacks of normal monsters too (the poison attacks of fairies of Ellin, elemental attacks of ToT's monsters...).
I'm pretty sure the attacks are coded differently in game as the poison immune card that u get from one of the normal fairy mobs make u "immune to poison attack" but in reality, that immunity is only given to the poison debuff inflicted on your character, rather than giving u immunity to post mist. Probably those you can check if your poison mist affected poison immune characters and if so, then maybe its down to coding
I checked out as you suggested: Spoiler: Ancient fairy's poison DoT With 47% partial resistance (F/P): Without resistance (NL): Conclusion: Ancient fairy monster poison debuff is poison magic. Spoiler: Ephenia poison mist Here is my failed attempt at today Ephenia boss In the video, at 3 : 39 the poison mist is cast. While I have Shining Fairy monster card and Partial Resistance, the 3000 DoT is not negated or cut. So I assume it's not coded as poison magic. And this is not related to the topic, but dying in Ephenia BOSS cost 10% exp... Hope this little system of elemental magic will be given some consideration to be completed.