Geralt the Witcher VS Demon Primarch (WH:40K + Witcher series)

[Elheru Aran]

Isn't Ciri a goddess for all intents and purposes as far as her power level is, IIRC she's actually more powerful in the books then in the game but with less control over her abilities, that's the impression I got from secondary sources I've not read the short stories themselves.

She can travel through space and to alternate dimensions approximately at will, is an excellent fighter, and has some command of magic (her education was cut somewhat short), but she's not a goddess.

Yeah, while those individual feats are fairly impressive, in terms of raw power she wouldn't be much at all compared to even a basic Chaos Space Marine Sorcerer.

[PREDATOR490]

By the end of Witcher 3 Ciri is able to teleport and move extremely fast at will. To the point she can one shot kill enemies in rapid succession before they can even respond. That said, Ciri is still striking her targets with a steel sword and while she is a good fighter, Witchers are trained to be highly agile. This kinda makes sense since they are expected to go up against any number of creatures so they are trained to avoid getting hit and wait for critical points to strike. They are not super strong, nor are they seriously more durable.
The only advantage Ciri has over Geralt is that her magical super powers allow her to augment her agility to the point she can 'blink' teleport to such a degree that clearing a room of multiple enemies before they can move is entirely possible.

Putting them up against a foe that has armour that can repel a steel sword strike would pretty much screw Geralt and Ciri. Ironically, Geralt might perform a little bit better since he has some offensive magic but Ciri can only run away.

In theory, Ciri could learn to become a Sorceress eventually which would make her capable of being a super-fast sword fighter combined with being able a powerful magic caster however that takes you well past anything the Books or Games have covered.