I find that even having armour over 8000 (also includes the other damage types around 200-600 each) I still take huge amounts of damage, I have been looking for amulets and rings that give one or more resistance types ( fire, poison etc.) I killed Abishai yesterday on gold, He is a unique boss being that he can use your own abilities against you, I am not sure if he uses the exact damage type as you regarding your abilities or if he multiplies the damage for when he uses one, the damage is massive.