Isshin is difficult no doubt, but I think his difficulty is way oversold. He doesn't do anything unfair and by the time you reach him, you should be fully aware of how powerful the deflect system is. Demon of Hatred and Owl (Father) are harder bosses in comparison because they have other gimmicks. Isshin however is basically a straight up test of most of the game's standard mechanics. Deflect, Mikiri, Head stomp, etc, it's all the same stuff, you just have to learn his moveset and timings like every other boss. Plus, the arena you fight him in is really generous in size and lets you stay just out of range of his long combos until you Mikiri the final thrust, which is telegraphed from a mile away. Seriously, if you've gotten this far, you can definitely do it.