Zangestsu, voiced by the ever talented David Hayter, is pretty fast, but telegraphs his attacks. Despite this, I'd suggest maxing your potions (9) and a few Ethers, and whatever food you might have (I had Rice Balls).
First, my loadout:
- Claw of Onmoraki (2h greatsword, from 8-bit coin), though there are way better great swords (Flamberge, for example)
You may want to try a faster weapon if you're having trouble.
- Pirate Hat
- Brigandine armor, a reward from a quest
- 2x Rings, if you're having a ton of trouble, try a Safe Ring, it lowers your attack by 10, but increases defense by 15
- Gunman's Scarf
Attacks:
Zangetsu can counter jumps by doing a slashing uppercut, so be wary of you try to jump over him.
He attacks by teleporting (usually to the far side of you), and slashing, slide forward/step to avoid this.
When there's a moderate amount of space between you, he may use his whip to pull himself up and toward you, before slashing downward. Easy to slide under him.
A simple, easily telegraphed swing. Back step to dodge.
Once he has about 50% health, he enchants his blade with fire, dealing additional damage.
A counter, he will gesture to you and hold his blade up. Lasts several seconds, you can use a ranged attack without being hit.
The fight is pretty straight-forward, and as long as you don't get greedy when attacking, and wait for openings, you should take him out without too much trouble.