After completing Spider Cleanup and Deliver Medicine, you will find this quest in the Adventurer's Guild.
Important note: if you accept this quest when Charlotte is not in the guild, she won't accompany you as an ally and she won't get experience from the quest.
Go talk to Jacob in his farm. The southern part of his farm then becomes available. The Young Avian is there.
To defeat the young avian without removing Aura's collar, there are some valid plans:
- If Aura knows Defensive Stance and Protect, you can use Charlotte to damage the avian and begin the battle with defensive stance. Aura will just have to protect Charlotte. When the avian takes enough damage, he will prepare an attack bringing him back to the ground and inflicting him with Exhausted.
- If you take all gold in the avian's area (look at the shining spots on the ground), the avian will start the battle with Enraged. In this state he only uses normal attacks and doesn't fly. The Light + Rampage combo is enough to kill the avian if Charlotte can boost Aura's ATK.
- Without Charlotte, angering the avian is necessary to hit with normal attacks. Don't use Rampage if you can't deplete the avian's 400 HP. Combining Light with normal attacks is longer, but safer.
Once you defeated the avian, go report to the guild to complete the quest.