In regards to the Witcher universe, and power of surrounding characters and creatures.
Witchers themselves by default posses the capabilities of about E8-11, in terms of spellcasting, basically capped off by telekinesis/tk-maneuver. However, that list narrowed only to the list I previously posted. Swiftblade might emulate Heliotrope to some extent, but I never went down the magic path to see how it really worked. Probably extra actions, probably not time stop. This is assuming he goes down the magic path anyways.
So then, you really have the last chunk of levels finished off by probably Swiftblade, an Alchemist PrC, or Initiator levels.
A small handful of mages posses the potential to be E17 with 9th spell capability. Witcher2 pretty much has an apocalypse from the sky event, combined with a greater curse. Though, these are largely ritual-time casting, and the mages only display the combat capability of maybe 5th or 6th level spells, as far as what they can sling as standard actions. Aside from the signs, other effects seen performed by mages are teleport, dimension door, flesh to stone, stone to flesh, shrink item, polymorph, personal range area protection from arrows, anti-unlife shell, summon monster, dominate.
You also have the Wild Hunt, which at the moment seems to be a bunch of plane shifting, nightmare riding specters, or invaders posing as specters. Home plane being alternate time flowing. This arc will probably be covered more in depth in a future game.
The final encounter in Witcher2 is about a CR14 - CR16
[spoiler]One on one fight, is essentially either a young adult, or adult gold dragon. Mages didn't get to rampage due to a suppressing mythal.[/spoiler]
A Witchers primary profession is to deal with unintelligent monsters, and in fact, killing intelligent living monsters such as trolls and dragons poses a moral dilemma. Of course, Geralt gets caught up in politics and whatnot, so he inevitably ends up slaying an armies worth of humans as well. Typically dealing with usual aberrations, magical beasts, undead, and demons, typically possessing no more than low to mid level SLA/Su effects if they have them. Monstrous threats are typically going to be large and brutal, or overwhelming speed & numbers, rather than the threat of a slay living SLA instagib.
Ultimately, if you try and dump Geralt into a 3.5e game, the disparity is going to be the magic capabilities of characters and creatures around you. Items similarly are typically enhancement bonuses, rather than producing impressive magical effects.