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I am currently playing SoA with Tactics Mod and blah,blah,blah....

So here is the Q:
Is this good combo and on wich level i should Dual-Class myself??

Oh and I HAVE TOB INSTALLED TOO

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Well, you really want to hold it off as long as you think you can or want to. The Wiz Slayer's MR only starts to sky-rocket after lvl 20, so if think you can actually restart with a lvl 1 Thief once you've already gotten that far with the Wiz slayer, then go for it. Otherwise, simply stick to lvl 13 as you would with any other fighter. You'll only get 13% MR (though that really isn't something to scuff at) and no HLA's, but at least you will have a decent THAC0, the best base ApR one can possibly achieve, Grand Master Proficiency level, and also the lovely 10% Cumulative Spell disruption. This will also allow you to regain your fighter abilities before things start getting really tough.