On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for the Otta, Son of Slugga side quest.
Rewards: 1,200 XP.
In the place marked on the image above, you will meet a man named Otta who has an extraordinary ability. Using his spiked mace, he can deflect stones with incredible power and precision.
Ask the man to demonstrate his skills, and then agree to a little competition. Using the bow, hit five targets - the same ones that Otta hit. After a successful attempt and a short conversation with the man, the task will be completed.