On this page of the Assassin's Creed Valhalla guide, you'll find a detailed walkthrough of the Vinland side quest: Breaking Teeth, Not Hearts.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet a man who is getting ready to hunt a terrifying beast.
Follow the warriors into the forest and help them hunt the animal. If you haven't acquired better equipment in Vinlandia, watch out for aggressive opponent's attacks and make frequent dodges. After the fight, the mission will end.