[Shwen]

Contains some spoilers
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I would say at lvl 13, by then you can get +++ in dual wielding and +++++ in katanas, or any other weapons (catanas are better in this case).At lvl 13 you also get some bonus attacks per round. You can level up your mage up to lvl 10 or higher in not time by lerning spells from scrols, so have your kensai collect them. then you can get some exp points for fighting monsters, solving some quests (i did the fighter stronghold), etc.
Why katanas? Celestial Fury, and that other one that gives you bonus spells, from the bonus merchant in Adventurer's Mart.