First off, the guide by Buda is spot on with tips on how to fight and you should definitely follow it. This is not to compete with that guide, simply to add some tips you may not know.
1. Do not approach this fight as trying to win at first. Look at it as training on each phase. Practice on Genichiro until you can beat him without healing. Practice dodging the black swings, learn the timing on parrying, learn the difference in his unblockable swing and thrust. You need to be able to counter his thrust every time. As you're learning his moves, if you beat him, let Isshin kill you and repeat until Genichiro is no longer a threat.
2. Now do the same with Isshin phase 1. Practice parry timing, learn the tells of unblockable swings and thrusts. You need to be able to counter every thrust. When he sheaths his sword, run away from him and to the left, then punish him after the double slash. When he raises his sword, spam B to back Up A LOT, his two sword slashes will miss you. Run away from his whirlwind attack.
3. You should be able to beat Genichiro and Isshin phase one without using a gourd. I felt like I needed all ten for phase 2 & 3 of Isshin because his attacks are so powerful and it's hard to tell if he's going to use a 3 hit combo or 5.
4. Now learn his new moveset. The spear CAN be parried, timing is just different. Practice till you can do it. Master the mikiri counter! Does major posture damage! Once I realized the reach of his thrust was INSANE, countering became a lot easier. I tried many combat arts on him, I thought that "Praying Strikes-Exorcism" worked the best. You can't spam it though, punish him when you dodge an attack.
5. For Isshin phase 3, learn the lightning parry or learn to dodge it, whichever you can pull off with confidence. Remember, phase 3 is close to phase 2 except he's more aggressive, uses the unblockable whirlwind attack, and the lightning attack.
6. Accept that you will die on Isshin phase 2 & 3, until you learn his moves. That way you won't get overwhelmed. Since you have mastered Genichiro and Isshin phase 1 though, it won't be as frustrating.
7. DO NOT use more than your first resurrection until you are on Isshin phase 3 and are confident you can beat him. If you do, you will have to grind out deathblows to fill you nodes back up. The first one refills at an idol, but the other two take about half a skill point worth of boosting deathblows to fill back up. If Isshin kills you twice just die.
8. That being said, a "Bundled Jizo Statue" will replenish a node completely! Unfortunately they only work outside of battle. Save these! If you almost have Isshin phase 3 killed, and you use another resurrection, these can be used to refill the node before your next try without having to boost.
9. You know those Dragon's Blood Droplets that heal Dragon rot? Well turns out they have another use. If you have the black mark across your resurrection nodes, you can USE THIS IN BATTLE to RESTORE RESURRECTION! This means that if you die and resurrect in Ishhin phase 3, then you can use a Droplet and regain the ability to resurrect. An example : You make it to Isshin phase 2 with 3 nodes. He kills you. You resurrect. You now have 2 nodes with a black mark. You deathblow phase 2. This takes your black mark away, and you are now on phase 3. He kills you again, you resurrect. You now have 1 node with a black mark. You use the Droplet, black mark goes away, giving you 2 resurrections in 1 phase!
10. Don't forget about your sugars! Yellow one helped speed up Genichiro and Isshin phase 1 for me, and blue one helped on phases 2&3.
Also, when an opponent parries you, it makes the same sound and flash of light that your parry makes. So attack until you see that, then get ready to parry.