This doesn't work with ranged weapons, you must kill the beasts with a melee weapon.
It also didn't work with the sword I used, I got it by killing everything with the hammer that was sold by the dwarf family's representative (Jotunn's Hammer).
Here's the list of animals you have to kill:
Main Area:
- Goat
- Crab
- Spider
- Wolf
Elves area:
- Mimic chests
- Flying serpents
Orcs area:
- Bandits
- Skeletons
I finally got it by killing all 8 in a single day using the hammer. I farmed them many times before with the crystal sword or whatever its name is, and it didn't work.