If you accidentily finished the story line and no not cross paths with another matron anymore, you can also:
- Select new Save slot from main menu
- Start a 'new' Lord of the Hunt campaign
- After the first obligatory mission (Torvin and Caragaths), do the "Matrons Rising" mission (South East of the map)
- In the second or third nest you'll encounter a Matron. Fire 4 regular arrows, then do a Fire Arrow (left control button).
- Achievement pops