He is incredibly powerfull, but a mage specializing in combat magic wipes the floor with him.

Overall witchers seem to be jack of all trades, some useful magic, some very good close combat skills, some useful alchemic skills. But a witcher won’t outmagic a mage, win a duel against some sword saint level fighter or brew better shit than some alchemist studying the craft for decades(granted, said alchemist wouldn’t survive witcher brews).