I agree with the guy up top. I'm a mage too and I can't do it. He somehow stops getting tanked by my pet and hits me with a crit. I got a 460 one time. I think you should complete ALL the side quests before doing this. I don't know, maybe level 15, my brother and I were thinking that maybe you should get that shadow ability to sneak up on him.
It may also be that your attacks are higher than your pet's so you need to wait until the attacks actually hit...
