You Can Only Get This One With Thorn.
- This Only Involves Merchants That You Find Out On The Road.
- You Can Buy Stuff From Them First, Then Rob Them.
- Since Robbing Them Gets You Into Fights, You May End Up With More Strixes Afterwards.
I Can't Remember The Exact Number, But 3-4 Merchants Should Get You The Achievement.
- Achievement Pops At The End Of Game, So I Would Avoid Letting Thorn Die Before Getting To Opacum.
Good Luck!