I'm not an expert, but personally I would target the mage and focus the shaman before the fight starts.

Purge every chance you get before they come rushing in. Nuke the mage when you get the chance, if you are lucky he will iceblock. At the moment they start rushing in,switch target to the warrior. Drop a fire nova totem, then shock him. This will usually drop his health low enough for the priest to use his pain suppression. At the moment he does that, NS/EM/CL+shock your focus and the shaman should die before they can do anything. After this, it should be pretty straight combat.