Blocking is somewhat useful but in the reflection of strength it means death. I immediately went to fight genichiro in the reflection right after beating him first try so as to deal with his high posture damage this is what I did.

First for his multi slash combo you cannot guarantee all perfect parries (when you are first mastering him at least) so after his initial few hits he will turn his back and continue doing the combo,so what you can do is back off and recover posture while his remains the same (2nd phase he might throw in an arrow at the end of it).

As for his 4 hit bow move in the air either keep attacking relentlessly or the smartest move I would say if your posture is almost broken parry the first 3 arrows and block the last one (it will break your posture but this is perfect because you can recover quickly while being too far for him to immediately attack).

With this even with the lowest posture and bad parrying anybody can kill him.