Don't know how much time you spent on this last boss but I can tell you we all went through that moment. Take a breath and when u finally get him, besides the unbeatable relief you will experience, you will realize that the biggest moment of frustration came after only a few tries. To get to beat bosses like Ishin or The demon of hatred, It demands to accept gettin buttkicked even after you start feelin at ease with a given phase. The foe brings new moves to the table and you need to learn them. But to do so, you need to do the first phases over and over again. In the end, it's highly probable that you will become a killer against Geni and Isshin's first two phases. And the final stand will be painful, your heart will beat faster, the sweat will make you slip and miss some controls,Isshin's health and posture bars will seem always to full, wheareas you will run out of gourds. You'll feel in a rush and start mixin up everything, but the final hit will come and you'll scream and feel your hands shaking. I finished my third run today and I still felt this way. And btw, I was about to quit for a while after my 10th death against Ashina Genichiro in my first playthrough. Glad I kept on. Hope you beat the hack outta him mate.