Hey I'm currently ranked 15 world in arcane (chozen) yet I've got pretty bad gear. Use incanters ward on this fight. Your primary goal are the legs,dont bothe with the one opposite side near the tanks. NEVER lose your 6 stack. Never dip below 60% mana. Only use IW on legs and avoid popping it before his crush (you'll lose 2seconds of IW stunned). Go to town with arcane missiles on legs.
Here's my log :
http://www.worldoflogs.com/reports/r...&e=9792#Chozen
Quick tip also for legs : Use a barage right before your alter time is about to expire, you'll pop right back to 6 stacks and you'll hit several targets.