Geralt is pretty strong and fast, but he's certainly not at the level of magic as a full sorcerer or wizard
Aard - Telekinesis
Igni - Burning Hands + (Fire DoT)
Yrden - Ground Trap, sort of Glyph of Warding
Quen- Vigor + Fire Shield, or other damage absorb + force feedback sort of effect
Axii - Short Dominate Person, Charm Person
Heliotrope - A few possible effects, damp power, minor time stop / short 3.0 haste
All Somatic, (no verbal?), often? visual display
Recharge Magic system
Brew Potion
Enhance Potion effects & duration, pretty sure there's a DR feat for that
craft(alchemy)
Enhanced alchemy?
Lots of skillpoints / skills... like... all
All weapons & many exotic weapon proficiencies. His swords are typically longsword. Lot of random weapons you can also use in the games.
light armor proficiency
Some ToB maneuvers
Evasive Reflexes
Power Attack
Flaw: Sterility
Mutagens are a bit unique.
Ultimately fleshes out a bit different considering the skill tree paths in the games.
Haven't read the books, so there's probably even more variation.
Can probably stitch together something from existing classes, but it would be better off to just have a homebrew class that mimics the choices in the character progression of the game, or other such legends of his capabilities.
Magic usually still rules in his universe, except certainly the more powerful magics take much longer to cast, seem to suffer recharge times, and can cause mental burn on greater magics. Mages only seem to be defeated when they decide to stick it out rather than teleport away.