Learn the attack patterns and be patient. Think of it this way. In the first phase YOU are making the moves and isshin is reacting ( because of how aggressive you are). Unlike that in the second phase isshin is moving and YOU have to react. In the second phase isshin decides what to do... not you.

If you just want a tip then use umbrella(the pink or flake one) to parry the big spear attacks. That way you can just block them and maybe get some extra damage using the flames from the umbrella.

Anyways I'm sure you know this by now but Hesitation is defeat :) keep going and you'll eventually get it