1. Aetherials have high fire resist, up to 75% for Herald of fire (used to be fire immune in beta)
Usually heroes have a tag within their name which hints at their abilities. Electrified - lightning, corrupted - aether, burning - fire, etc.
2. Maybe reflective? They have skill which acts like Mirror of ereoctes - makes you damage immune and reflects all damage taken.

I have seen mod on Nexus which lets you track damage taken and see what killed you, try it.

In addition to Gremlion's points, if you're level 85 it's possible that you've reached Nemesis status with a faction. Nemesis monsters, while beatable, are immensely powerful and some of them could one-shot you if you don't have appropriate resists.

Most elites (yellow names) aren't really strong enough to worry about. Most hero monsters (reddish/orangish names) do have a type under their name, sort of like affixes in Diablo. Most bosses (purple names) don't have affix info but they do what it looks like they do (they're custom-made, unlike heroes and elites which can be randomly generated). Most Nemesis monsters (dark red names) will just eat your face.

Where did you find artherial were resistant to fire? Is this documented somewhere?
If I encounter the Orange named that was killing me again I will tell you.

Game strongly hints at that.
Aetherfire items, monsters, which use fire skills...
I can think of one notable elite killer, though.
https://grimdawn.gamepedia.com/Loxmere_Nightmage

It's exactly him!
I'm just meat for him :p

He has nullification, which removes all your buffs and does damage based on your current amount of energy, then shadowstrike, which does normal shadowstrike damage (like x4 of normal attack).
Problem is that people usually start running away, which lets him refresh his skills, you need to man up and tank him.
And, since he has a single spawn point, you can simply avoid him.