After you die, there is a hidden loss of your natural resistance if you have done the Anya quest. If you are in nightmare and did both normal and nightmare Anya quests for +20 to all resistances, then that bonus was missing after you died. That made it 20% easier for the lightning attacks to kill you. I say "hidden" because the character screen does not show this loss to resistances when you die. You only reset this when you leave/enter the game.
Those gloams are a general problem. It may be easier to start a new game and hope for a different/easier set of monsters than to fight through them. If you think you just HAVE to figure out a way to kill them, max lightning resistance is the first key. A single point in salvation for the paladin works wonders for the team. They move around pretty quick too, so if you can slow them down with any cold spell...blizzard, orb, frost nova..., that will help.