ScottG has it pegged, dual wield Katanas, Celestial fury, and the Zerth blade. By the time you get up there you will be a killing casting machine.
BTW, I believe that there is no benefit with waiting to lvl 13, dualing at lvl 12 gets you the same amount of prof points. It also is going to seem like forever, getting your Kensai abilities back. I have a kensai/mage that I dualed at lvl 9, and he is unstoppable.
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