I'm playing Witcher 3 so I guess it depends on whether Gerald sees him first and whether Nemesis has a bunch of guns. If he has no guns or geralt sees him first I'm pretty sure Nemesis dies. Magic, potions, poison, I'm pretty sure Nemesis loses in a one-on-one fight
Edit, I was thinking nemesis from the movie not from the game so now that I have been updated on exactly how ridiculous he is I'm thinking it would probably take more than just geralt but I'm pretty sure yen and triss and all the sorceresses could take him down