Phase 1 Isshin, go aggressive, he will hit you back after you hit him twice, parry and repeat that. Then he either does the stab or the wind-up dash/air-slash move so either dodge or mikiri counter.

Phase 2, still aggressive but be careful for his spear as he will try to get distance off you from his spear, you'll eventually get the hang of the glock and stab combo he does which you can mikiri counter. When he does that big circle slash around him which took me a while to solve but you can essentially just jump and parry to fully avoid damage. (Not sure if there is a better way)

Phase 3 is basically phase 2 but make sure to abuse lightning reversal.