I played a character heavily inspired by a Witcher once- a hexblade who carried a large number of potions, poisons, and miscellaneous oils/silversheen/etc. I was modeling her after Geralt in the first game, so I took power attack (to represent the "strong" style), dodge, mobility, combat expertise, and spring attack (I was going to take whirlwind attack, and use it with combat expertise to represent a "group" style; I figured "fast" style would be essentially normal attacks, since they're the most accurate, and hence the best for hitting nimble opponents). She had a lot of ranks in craft: poisonmaking and cross-class ranks in various knowledge skills; wielded a one-handed sword for versatility, etc.
Basically she was all about adaptiveness. What ideas do you have to make a more accurate 'Witcher' though, how would you stat Geralt?
Things to remember:
-Witchers are all about mobility and rely heavily on dodging and parrying; they wear only light, unrestrictive armor.
-Geralt seems to wield a longsword.
-Witchers are immune to disease, heal very quickly, and have great stamina and resistance to poison.
-Geralt knows and uses a very small selection of spells, which include powerful, concentrated [gusts of wind?], conjuring small blasts of fire, mental domination, forming a damage-absorbing field around himself, and creating magical, debilitating traps.
-Geralt, and Witchers in general, are highly knowledgeable about monsters of all kinds, herbs and potion-making, poisons, magic, and probably a wide variety of other things.
Edit: Geralt is also very good at hand-to-hand fighting, and has very good senses.