I'm not fully sure about this but I'm betting the mobs with affixes have extra % to hit you with extra damage to negate your resistances just like we do. Not to say he should be one shotting you but most elites/rates are going to consistently hit harder with their affix.

That particular boss is a hard hitter, though seeing your res, they are low, very low, even for that level of the game. How much HP do you have too ? Might want to boost them a bit. Try and get the Salvatory anchor node and put heavy chest and head pieces to get the most benefit. Respec if need be.

My life pool is 2k. Dang just did one. Do you know the way resistance works? Is the valve of the resistance blocking that much damage from said damage type? The defensive options seem far and few in between and are pretty vague. I'm trying to do a rend build so I went primary Wisdom and secondary Ferocity. Should I have done toughness as my second option?