This fight is made pretty easy thanks to Zangetsu joining you for it (took me roughly 5 minutes?). Depending on your playstyle, there are a few ways to go about this fight.
That said, still bring a few Potions because it can hit hard, and some Ethers.
If you prefer ranged (either using a gun or ranged shards), then just stay back and get behind Zangetsu when he puts up his blade (like he does when he is attempting to counter), and he'll deflect most/all of the bosses projectiles when he launches them. Then just keep hitting the boss until it dies.
If you prefer melee, use whatever weapon you're most comfortable with (I used the Flamberge) and preferably the "Beast Guardian" Manipulative shard dropped by Wolfmen. Whenever you see the boss rev up for an attack, use this to negate all damage.
Following the melee method, I only had to heal a few times because I wasnt familiar with the bosses attacks, so it should be too difficult.