i used a pally and 4 elemental shaman and I couldn't do it (at 70) until I converted one of my shaman to resto and dedicated the resto shaman to only casting heals. I don't think I was using magma totems back then
I would suggest a dedicated healer. if you do a good enough job of holding aggro on your tank, then hopefully you only need to cast most of your heals on the tank.

get as much set & forget AoE going as you can ( you can use arcane explosion but I would only use it in a pinch ). I'm guessing you need to use flame strike.
my shaman use magma totems ( so much easier to cast than flame strike ), for my pally/shaman I have the hole team on top of the boss so the concecrate and the magma totems burn through any ice blocks or adds I get hit with.

the healer should probably have a heal the tank macro and a group heal macro for a minimum, the max would be a seperate heal button for each character.