Make sure you drop your tremor totems - those are a life saver.
I usually position my toons close to the wall, droping those tremor totems and then blow heroism as soon as she becomes immune and kill that add asap.

For a mixed group it's a lot harder since what usually kills you are those fears in combination with her 5k damage on random toons. You really have to heal through her damage and have the tank focus on the add.