This is a part of Iris Gem Quest. I do this after finishing the Water Trial. Here is what you need to do:
Speak to Rose on the field, choose the option which talk about the robber kid’s condition from the Volgran Forest.
Now head to Lastonbell and talk to the merchant near the exit that leads to the Meadow of Triumph, and then talk to Rose again, and search the topic which make her ask you to accompany her to Volgran Forest.
Head to Volgran Forest, you should see a purple star on your map now. Head there to trigger an event and fight this Hellion.
Recommended Level: 41-45
Recommended time frame: After finishing the Water Trial
Use Sorey’s freezing strike to deal quick power hit damage on this boss. This boss is very easy, no need any specific strategy :). You will get Purple Iris Gem: Do.