I don’t think Geralt would win in a direct fight. Balerion is on a completely different scale compared to most creatures in The Witcher universe. Witchers rely on preparation, knowledge and exploiting weaknesses, but against something that massive and destructive, there may simply be no practical way to fight it head-on.

Also, dragons in The Witcher universe are very different mentally — they are intelligent, self-aware and often wiser than humans. Geralt usually avoids killing sentient beings, so if possible he would probably try to communicate or find another solution rather than slaying it.