This page of our guide to Ghost of Tsushima has a description of the battle with Bettomaru - this boss appears in The Undying Flame side mission.
Bettomaru is the last boss you'll have to face in Mythic Tales.
The fight with Bettomaru takes place at the very top of Mount Jogaku. Try not to lose resolve during your way up. With it, you can heal the hero or perform special attacks.
Bettomaru isn't much different from previous opponents. You can test a flaming sword on him. It's a new skill you've acquired recently. Try to hit the boss with the flaming sword to inflict damage that will deplete his health bar over time.
If you have full resolve, you can use the Dance of Wrath before you get to the boss. Thanks to it, you will defeat this opponent with just three strikes.