You will come to this boss once you reach the top of Ashina Castle. After defeating Ashina Elite - Jinsuke Saze
, you go out the window and grapple to the top to begin the fight against Genichiro.
This is a three-phase fight. The best tip I can give for this fight is to stick close to Genichiro and be aggressive. Attack when there is an opening, then when he deflects go on the defensive. Keep closing the distance so he can't use his bow. You want to fill up his Posture gauge as he has too much vitality for the battle to go on forever.
The best strategy is to just block his attacks, and eventually he will go for a jumping overhead. Deflect this, and he will immediately go for a thrust attack. Use the Mikiri Counter by dodging into the thrust for some serious posture damage. If you need to heal, be wary as he loves to pull out his bow and take some pot shots at you.
Once you take both his lives and think you've killed him, he takes off his armor and summons lightning for the third phase. Here do the exact same again. Be aggressive, when he deflects you, deflect his attacks and keep attacking.
The only difference is the last phase is the addition of lightning attacks to his arsenal. There are two methods to dealing with these attacks. The first way is to just dodge sideways as the attacks are mostly frontal. This is the safer option. The riskier alternative is to jump into the air and hit
as the lightning hits you for a deflect and some huge posture damage. If you can pull it off it makes the third phase really simple. Take down his last life bar, and claim this achievement.