The Champion Paladin Arcanist armor gives you 13% Resistances against any element used by an Undead enemy (13% Resistances at Lvl. 150). For Demon enemies, it says that the effect is 2/3* as effective. I assume 2/3 means it's 66% as effective. So what would the Elemental Resistances be against a Demon enemy?
In subtraction, 100% is the minuend and 13% is the difference, the value used by balance standards is the subtrahend, to find this value subtract the difference from the minuend, multiply by 2/3 and subtract the new subtrahend from the minuend to find the difference against Demons, 2(100-13)/3 = 58, afterwards 100-58 = 42% resists.