I had the most problems with those Cultist boss with his powerray or whatever it is. One has to heal all members or else he kills the backrow-members. Haven´t made it to lvl 9 though yet.
I am currently trying to finish hardcore without a party member dying. Currently i try Cleric, Ranger 2x Mage. I might switch to 2x Cleric 2x Mage. I would recommend to get rid of the white/grey stuff. It´s just annoying to throw all of it away. Also the game loses it´s challenge after lvl 9 quite a bit.