This page of our guide to Ghost of Tsushima has a description of the battle with Altan - this boss appears in The Message in Blood side mission.
Message in Blood is the last side mission commissioned by Yuna. During this mission you have to fight many enemies in Mongol camp. However, at the end there is an opponent that has to be defeated in order to finish this chain quest.
Altan looks like a tough opponent due to his strong armor and shield. But you don't have to worry about it. If you approach him properly, you won't feel any difference.
Altan often covers himself with his shield. It's a good idea to strike him with special blows that easily pass through his dodges.
It is worthwhile to protect yourself before entering the boss area and recharge your Resolve (represented by the yellow circles above the health bar). This way, you will be able to heal yourself or inflict damage with special abilities. Try to obtain as much Resolve as possible by killing the ordinary opponents which you can encounter in the camp.
If you will fully recharge your Resolve, then you can use the Dance of Wrath before you reach the boss. You'll defeat your opponent with just three strikes.