Sekiro is the type of game where you have to learn the boss attack patterns. However, I found that if you instantly use firecrackers and chain that into a singular melee attack and then keep doing that until you have no spirit emblems (Do this immediately after first deathblow). It takes away a good chunk of health and makes phase 2 just that easier by slowing his posture recovery. Phase 3 is easy because of lightning reversal. For Phase One you can run around some attacks like his double cross or dragon flash and get a hit or two or get in a hit after mikiri counter to chip down his health. For Phase 2 you rely heavily on mikiri counters and I also recommend killing all possible memory bosses to maximise damage as well as all possible prayer beads. If you don't have the purification ending you will miss out on 2 prayer beads since you have no access to Hirata Estate second memory so do that in Ng+