This guy is a horrible pain for caster groups. For a largely shaman group, I'd say wait til 68 and drop 4 Fire Elemental Totems, then have 2 Shamans focus on healing the tank while the other two cast their long ass lightning bolts. If/when they get hit with a fire aoe, switch them out and have the two healers cast bolts while the other two chain heal. Cast your best shock when they're up. When I did this fight, I had trouble too, and had to pretty much rely on DoTs and instant casts to do any damage while kiting.