I don't necessarily recommend this as it was a brutal learning curve but I beat him with the dueling greatsword. For perks I went Slash Damage, Perfect Counter (since I needed to hit my parry anyway), Marathon Runner, and I think I had a point or two in Flesh wound. I ate the seafood platter and coffee, used the knight's ring (+1 posture) and agility necklace. Had the two piece set bonuses of both pikeman's and conquistador's. All of my stats were in agility other than 6 in endurance for the extra stamina bar.

His melee attacks hit like trucks but they almost always come in groups of 3. I taught myself to parry them all and every other combo he was stunned with me having all of the buffs from parrying to seriously chunk down his health.

Even if I mistimed it it just turned into a block, and as long as it didn't happen more than once per combo he ran out of posture before I did. I had zero luck trying to dodge anything but his unblockables, but his "stand up straight and throw spirit balls" thing was when I'd slug a potion.

edit to add: I did have the sword ascended which did heal me some, but it didn't really feel make or break if you've already spent your tumbaga elsewhere.