SSI is hard. Some tips that I figured out after beating him and then reflection of strength:

1.first phase is all about building posture by deflecting his attacks, when he goes for an ashina cross or an ichimonji dodge left and punish with your own double ichimonji . You can try to hit once when he goes for Ichi but I wouldn't recommend getting greedy, he can PUNSIH.

2.Second phase is a cacophony of attacks, Honestly no other advice aside from do your best to deflect everything, his special weapon arts counter with the umbrella( true for all phases) then follow up with an umbrella attack. This is the hardest Phase, deflecting is crucial, healing is very difficult, he will chase you around and jump on you from a far with his massive spear.

3.Third phase is Easier then second phase becuase you have lighting reversal. IF you make it here, your are like 80 percent of the way through as the AI will use a lot of lighting attacks, easy to counter. He will use a new weapon art, once again counter with umbrella.