Physical and Elemental Resistances

I have a couple questions regarding the physical resistance independent from pdr that Empress, Magnus, Hilla, and the new Root Abyss bosses have.

I realize certain classes are immune to it, though I'm only aware of Evan, Archmages, and Luminous. Are there other classes immune that don't face elemental reduction? Only those classes

Also, Is it a flat 50% reduction, or does it take into account a classes' % Damage and % Boss buff, or perhaps is it affected by any other variables I'm not aware of? For all cases, damage is halved. Any DoTs and summons also get halved.


Thanks in advance.