That mage-dude in nexus is a f.... pain btw. Not hard, just annoying!
Are you talking about the boss (http://www.wowhead.com/?npc=26731) or about the mage dude trash mobs who ice-block your tank and sheep randomly?

Because I tried to fight Grand Magus Telestra and got owned badly - I barely came to 60% and then my healer died (priest with ~7k life buffed) .. any tips?