- Level a bit (easily done with some backtracking) until you are level 9 or 10 or so
- Backtrack a bit behind the safehouse. You'll find a
farmer, hand him the
Rice Seed- Do the sidequests of the lady in the staircase near the Alchemist and the Shop. (She asks you to kill a certain demon type, after you've done it about 4 more of such quests show up. They are easy to find in the castle, for one of them return to the shit)
- One of these will net you 3 full healing items + Max HP boost and some other helpful items (hold on to these for now)
- After you get the healing item return to the farmer. He'll have your rice ready.
- Go to the Alchemist and craft the Rice Balls (2 of them)
- Eat one Rice Ball for an easy Max HP boost
- Take the other Riceball to the shop. At the end there's an old lady asking you for food
- Give her the Rice Ball, she'll give you the
Thorned WhipThe Thorned Whip is the most effective weapon because it outranges Zangetsu's sword. Try to get behind him or stay in front of him.
If found that running from one edge to the other works best. Do note that Zangetsu has a dagger to throw as well but that only does minor damage.
Learn the jump attack cancel. This is pretty easy. Do an attack right before you land from a jump and once you land you can do another attack.
Do note that recovering from a Whip slash takes a bit long so only do one hit at a time if you fail the jump attack.
The fight has two stages. If you're low on Ether I would recommend to hold on to your magic until the second stage.
The second stage of the fight is the toughest, he'll have a flaming sword and more HP. Repeat the above steps in terms of fighting strategy but keep a bigger distance, especially from his jump attacks
which end with an explosion. Use your healing items wisely and Zangetsu is pacified.

If you're more of an aggresive type you can also buy the 'Estee' in the shop, which is a stronger rapier. The rapier has the advantage of landing 3 hits before Zangetsu can do one hit.
However, this is only recommend if you've mastered the backstep.
As far as Shards go, I found the demon hound summon and shockwave very effective and the higher grade True Arrow. Mostly because the animations are quick.
Something like the Fire Canon takes too long to travel and the Tentacle thing is rather slow.