Khotun Khan (Boss) | Ghost of Tsushima Wiki

Khotun Khan Boss Guide First Encounter:

 

 

In this first encounter, you can't defeat Khan; even if you somehow manage to take out all of his health, he will still keep fighting and never die. This means that you won't have a full-fledged duel with the leader of the Mongol army. The only thing you can do is to try to prolong it a little bit.

Khan will block your attacks and use his "red attacks" - they are very difficult to avoid even by dodging. The fight ends when Jin loses and falls off the bridge.

 

Khotun Khan Boss Guide Second Encounter:

 

 

Khotun Khan wields a spear, so Wind Stance will be effective against him. 
By now, you should have mythic techniques such as the Heavenly Strike and Dance of Wrath. Use this to your advantage when you have enough Resolve nodes but keep an eye out and make sure you have enough in case you need it to replenish your health. 
Patience is key to winning this duel! Whenever Khan will use an unblockable attack, dodge it. Also, learning the Unyielding Spear Parry will allow Jin to parry blue attacks of the Khan, then follow up with a counterattack. 
Whenever the Ghost Stance is also available to use, do not hesitate to pop it when the meter is full! 
When Khotun Khan's health is about nearly empty, he will fight dirty and throw some poison at Jin, then flee to his ship and call in two guards to cover his escape.
Quickly press the down button on the D-pad to cure yourself. Kill the two brute enemies and then chase after Khotun Khan. Along the way, you will encounter more Mongol guards who will try to slow you down. When you reach the Mongol fleet, another cutscene will trigger. You'll now need to fight and get past through the Mongol guards on the ship until you reach Khotun Khan. 
This time, it won't be a duel, so you're free to use your tools here. But remember to switch to Wind Stance once you're facing Khotun Khan. After enough damage, Khotun Khan will stagger back and call in more reinforcements, switch stances accordingly depending on the type of enemy you are facing, and don't forget to use your tools. 
Most of the guards on the top deck will try to throw explosives at you, dispatch them first by shooting them with your bow and arrow. One to two shots should do. 
Now, kill the Mongol guards first before shifting your attention towards Khotun Khan. After enough damage, Jin will shatter Khotun's spear and he will switch it out for a sword and shield. Adjust accordingly with Water Stance as more guards arrive. Also look out for the archers that are positioned on the upper deck. Just shoot them down with your bow and arrow. 
After enough hits against Khotun Khan, you'll be prompted to press the △ button to break his shield and to kill him.