1) Go to the cabin and take the knife.
2) Choose the options to help the princess escape from her chains.
3) The door will get locked from outside and you can use the knife to cut her arm to unchain her.
4) While trying to leave, the narrator will take control of your body and will force you to attack the princess.
Scroll down through the options and choose to warn her.
5) She will dodge and this time choose to slay her and you'll get the achievement.