Well, I'm gettin kinda tired with my Paladin (stonghold quests are lame), so I've decdied to start over as a Kensai/Mage. What level do you suggest dualling him over at? ans since the will be a while when i can't use my fighter skills, what proficiencies should I take? Oh, and what are the best wepons for this character? Thanks!

oh, about 15 I would say in the long run, though most people duel them straight away. I would take the usual...Katanas, swords, staff if you plan to have your mage at the front.

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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.

lvl 13 is indeed the optimimum lvl to dual , if you have ToB.
No idea what the best lvl is for SoA only, sowwy :(

Thanks guys... anything else i should know?

Take katanas, and get the bonus edition merchants if you haven't already. This gives you Dakkon's Zerth Blade, awesome for k/ms.
The optimal lvl in SoA would be 9, that way you can hit 17 in mage.
IMPORTANT: While you're still a kensai SAVE ALL THE SCROLLS YOU CAN. You can get three scroll cases at the scroll merchant at Waukeen's Promenade, the woman at the adventurer's mart, and the Copper Coronet. That way when you dual, you can gain xp quick. Also, check out the k/m guide at Gamebanshee.com, I don't know what's up with it though, I can't get it on my IE. Oh well.