"Heartbreaker"
Seduce Ves.
At the end of chapter 2 make sure that you select Roche's path to continue the story. Doing so will allow you to access the camp and explore it.
During your time in the camp you will hear about arena tournaments and that the king holds them from time-to-time. You will need to complete the quest "The Butcher of Cidaris" which will open up the arena for you. From here you will then need to beat everyone in the arena, and your last opponent will be Ves.
Once you defeat her she will tell you to meet her later on. If don't talk to her soon enough you will bypass this part of the quest and miss out on the achievement.
Once you encounter Ves outside her tent you will then need to talk to her. If you select the right options this will then lead to you sleeping together and unlock the achievement.
The following video will show you her location and what options to select. This video has also been edited due to youtube restrictions.