This is a MISSABLE TROPHY. So when you go to meet Damian at Caunt de la Croix's mill save your game. Or you can save the game in the quest "The Night of Long Fangs" when you get the choice to either save Syanna, or go find the Unseen Elder.
What you want to do is to choose the option to save Syanna. When you get to the land of fables you with have an option to speak to the little girl who sells flint. Don't do it. If you do and get the ribbon this will save Syanna when you confront Dettlaff.
Just get the three beans and plant them. Go up the stalk and kill the giant and then go and confront Dettlaff. Now that you don't have the ribbon he will kill Syanna. Then choose to let him go. You'll end up in prison and the Trophy will pop.