How do you kill Sword Saint Isshin Ashina in Sekiro: Shadows Die Twice? Depending on which ending you've pursued, this will be the final battle in the game, and you'll need to bring your A-game if you're going to take him down. With a fight consisting of four stages, this is the longest tussle in the whole game. Power yourself up as much as you can, and you may stand a chance.
Sekiro: Shadows Die Twice - How to beat Sword Saint Isshin Ashina
Sword Saint Isshin Ashina's boss fight actually begins with a rematch against Genichiro, Way of Tomoe. This is a repeat of Genichiro Ashina's lightning-throwing form from earlier in the game, only he'll have a new, sweeping attack with an enormous range. You can dodge it by either running away immediately, or move directly behind him. He won't use any lightning attacks here, so you don't need to worry about that. Stay on the offensive, parry as much as you can, and he'll go down pretty quickly.
Then it's onto Sword Saint Isshin Ashina himself. The first phase isn't too bad, although he has some attacks with huge range on them. The Mikiri Counter will be your best friend against this boss, as he likes his thrust attacks, and countering them deals huge posture damage. Try to dodge around him for most of his other moves to get some sneaky hits in.
It's the second and third phases where things really get tough. The difficulty spikes here as he gains the use of a huge spear and a gun, but there are ways to deal with him. You can use the Mikiri Counter on his thrust attacks, and jump above his sweeps and jump off him to do some more posture damage. We found the Shinobi Firecrackers to come in handy when he's charging up his big sweeping spear move, which is otherwise hard to avoid.
Sometimes you'll need to get away from him to heal, but be careful when you do. He'll use his gun or do a leaping slam attack with a lot of range if he catches you healing up. For the majority of the fight, you want to stick close to him as much as you can. The Mist Raven Prosthetic Tool can make your life a little easier, but we'd honestly stick with the Shinobi Firecracker for this fight.
In his third phase, he'll begin using lightning attacks too. You'll need to watch out for this, as you can counter the lightning in the same way as you did for Genichiro, which deals lots of damage. Jump in the air just before he lets the lightning loose and press R1 to send it back. Aside from that, it's a repeat of phase two. Losing at any point in the fight will send you back to the very beginning, where you'll have to beat Genichiro again. This is what makes it hard; it's a very long battle, but with the usual tactics of aggression, sidestepping, deflecting, and countering, it's not insurmountable, and can be over surprisingly fast once you know what you're doing.
Beating Sword Saint Isshin Ashina rewards you with the Saint Isshin Memory, the Dragon Flash Combat Art, and 20,000 XP.