Unlike the other games in the Yakuza series to fight this Amon you don’t need to complete all substories. To fight this Amon all you need to do is complete 2 specific substories. The substories you need to complete are related to Sodachi (The owner of the dojo saejima could run in Yakuza 4 and could fight in 5) and Saigo (The person who trained Akiyama in Yakuza 4 and 5). I’m gonna quickly go over their substories so you can unlock the Amon fight which takes place in the final chapter of the game.
Sodachi- Sodachi substory starts in chapter 1 but you cannot finish it til chapter 6. In order to start his substory and you must first complete “#23: We are Committed to Results” which is the Rizap gym substory. After completing that substory Sodachi will challenge you to a fight. After beating him in that fight you will now have to wait til chapter 6 to complete the rest. Once you’re in chapter 6 Sodachi will be roaming the city and you will have to fight against him a few times. After winning a few times you will have to meet him in Children’s Park and defeat him one last time to complete the substory.
Saigo- Saigo will appear in chapter 4 laying on the floor right by the Taxi at the ropeway in Onomichi. When you talk to him he’ll ask for a specific bento. The bento you need is called “Bento Lunch Set (Pork)” which is not sold anywhere in Onomichi. To get this bento you need to buy it at the Poppo convenience store near the Yoshida Batting Center in Kamurocho. Once you’ve given Saigo the bento lunch set you’ll have to fight him and win. After you’ve won he’ll be wondering about Onomichi. Look out for a purple dot on the map. If you follow the purple dot you'll see him and a notification over his head. Now you have to fight him 2-3 times. After a while, during the day during chapter 5, he will go back to where you gave him the Bento Lunch Set and you will have to fight him one last time.
Once you’ve beaten both of those substories and have reached the final chapter of the game Amon will send you a text message saying the time has come yet again. This fight might only be available at night so heads up if you’re doing this in premium adventure. Now that you’ve gotten the text and the substory has begun you can take a taxi to Amon’s Island. Before we do any of that if you want to make this fight super easy we’re gonna do a few things first. First, we’re gonna change the difficulty to Easy just to make this super easy. After doing that, buy some tauriners+ or Staminan XX at either city's pawnshops cause we’re gonna abuse the Extreme Heat RT ability. Last, of all we’re gonna save then go drink a CC Lemon, which you can get from the BOSS vending machine at the bottom of Nakamichi Street which will give you an attack up.
Now that we’ve prepared, get into a taxi and head to Amon’s Island. A cutscene will play for a bit and then the fight will start. For a majority of the fight we’re just gonna use the Extreme Heat RT ability and depending on what you want to do you can either just keep spamming punch at him or try to use Essence of Repeating Knee (If you have the ability unlocked) by holding RB and Y to kick him and try to get the heat symbol to appear and then do the QTE to do more damage and just keep spamming that move. When your Extreme Heat wears off immediately use a Tauriner+ or StaminanXX and immediately go back into Extreme Heat. Keep doing either method til you see Amon put an orange shield around himself which makes him take significantly less damage. This shield doesn’t mean much since you can basically still just spam attacks in Extreme Heat until he dies. Hopefully, Amon should be dead by the time all the drones and Roomba’s swarm you and explode on you. After the fight is over you’ll unlock your achievement and a cutscene will play and you’ll unlock a rare variant of Amon for Clan Creator.
This will the last game we see Kiryu fight, Amon, unless you want to count Kenshiro using Kiryu’s outfit in the Fist of the North Star Lost Paradise game.