I find that the best guarantee is to pre-heal and suffer massive overhealing, not reactive healing on each character with a separate keybind. If you have an AOE heal, spam it regardless of if anyone is taking damage. If your healer isn't capable of sustaining the mana for this, you need to gear for MP5 until you are. This fight can become a very mindless tank and spank if you pop frost resist aura and just mindlessly spam healing. The time you spend moving your characters around might be better spent healing since, as you said, the frostbolt is really the thing that nails the coffin on you in the end.