How do you kill Genichiro Ashina in Sekiro: Shadows Die Twice? This boss is one of the hardest in the game, but not necessarily because of his attack patterns. Once you've delivered two Death Blows, he'll still be alive and kicking, and after a brief cutscene, you'll be back in the fight and he'll have an all new range of attacks. This fight will test everything you've learned about Sekiro's combat, as well as introduce some new stuff to deal with for good measure.
Sekiro: Shadows Die Twice - How to beat Genichiro Ashina including Way of Tomoe form
After the cutscene plays out, you're in for a long fight, so make sure you come as prepared as possible. Make sure your Healing Gourd is as upgraded as it can be, and bring other useful items like Pellets and the different Sugars to buff yourself. We'd recommend using the Mist Raven Prosthetic Tool so you can get out of dodge if you're not confident about an incoming strike. Since you faced him at the very start of the game (this is the man who chopped off your arm), you'll have learned a lot about deflection and how the combat works, and the fundamentals really are the key to victory here.
Play aggressively. It's important to stay on him as much as you can, but remembering to dodge or preferably deflect attacks when they come. If you get too far away from him, he'll use his bow to fire arrows at you, which can be very dangerous if they connect. You can try to counter this with the Loaded Shuriken Prosthetic Tool. He also makes use of some unblockable attacks which you'll need to watch; sometimes they're thrusts, sometimes they're sweeps. You'll need to try and react accordingly.
You need to make it through the fight with as many of your replenishing items as you can, because even when you Death Blow him twice, you're still not done.
After another cutscene, he'll take a new form: Genichiro, Way of Tomoe. Here, he's stripped off most of his armour, which makes his posture pretty weak, but he also wields lightning as a weapon, which can be a devil to deal with. Again, the trick is to go on the offensive. When he throws lightning, make sure you're in the air; if you're on the floor, it's going to deal heavy damage. You can counter the lightning by pressing R1 at the right time, which will send the attack back at him, which stuns him and does a number on his health.
Once you do eventually take him down, you'll earn the Genichiro Ashina Memory, Bloodsmoke Ninjutsu skill, and 2,000 XP.