This achievement is available on the Roche path in Chapter 2, during the Ave Henselt! side quest.
Begin by completing the The Butcher of Cidaris quest in the Arena. As you win fights, Ves will volunteer herself as your opponent. If you defeat her she’ll invite you back to her tent in the Blue Stripes’ camp.
Go and visit Ves in the camp and give the following responses during conversation:
- That was a great duel
- I also feel like using this opportunity
- She’s both brave and beautiful
- I like that idea
The achievement should pop as soon as you pick the last dialogue option.