A very important part of this fight is dispells. Is your priest mass dispelling everything? Kiteing the warrior is a good thing and it is like a pvp fight, but CC is very important. Genrally we banish the tree, kill one heals, then work our way thru the DPS by armor class. With your setup, cc the druid, kill shammy, throw out other CCs to start and get everyone else to use their CCs for self defense. Yes they will become immune for a moment if you CC them over and over, but if you CC for self defense then they shouldnt. Like if the paladin come after your mage have them sheep it. Have your hunter throwing down frost traps to slow everything down ect. You want to use everything you have to stay alive, just like in PVP.