Master Carpenter on Nightmare as Zangetsu.

[EarthenSpire]

Has anyone been able to beat him on Nightmare difficulty yet? And if so, mind giving some tips?
It seems like you just have to hope he never builds a demon since it's basically impossible to dodge once he does and you die in 6-8 hits. He also has 6666 HP so you can't just damage rush him.

[Emerald Lance]

If I recall correctly, he only builds a demon if you attack him while he's carving, otherwise it's a chest. It's been a hot minute since I've fought him, so I could be way off.

[EarthenSpire]

I read about that on the wiki and tried that, however, it seems like that's only the case if there's already a demon summoned. I tested it by going into the fight and not hitting him once until he started crafting and he still crafts a demon. The fact that chests don't exist in Zangetsu mode could be affecting it.

[Emerald Lance]

I hadn't thought about chests not existing. I haven't gotten far with Zangetsu yet, so I'm working off of Miriam logic. Other guess would be to make liberal and smart use of the Akuma dodge, but that's more of a general strategy; I can't think of anything specific to do against this particular boss. Sorry. ;_;

[EarthenSpire]

Update: I finally beat him.
Turns out, after he's summoned two demons, he won't summon any more for the rest of the fight. So after he summons two, you can spam your uppercut attack which has invulnerability to kill them.
After that, it's just a matter of constantly jumping over him to cause him to miss his chisel throw and attacking him from behind while he's turning around.

[HSuke]

He definitely crafts chests in Zangetsu mode. Here's one with 2 chests: https://imgur.com/a/myYrZpq, and I've seen him summon 3 chests in a single round.

Unfortunately, the chests don't do anything, and he can still summon a total of 2 demons afterwards. (I've also seen him summon 3 once when he somehow summoned 2 at the same time).

A very hard boss. I think the easiest way to beat him is to jump over him, hit him 1-2 times, estimate when he's going to throw, and then do the uppercut attack. You'll need to take out the demons quickly with repeat uppercut attacks. I only attempted and beat him at Lvl 40. I couldn't imagine beating Nightmare mode, so you're pretty damn good.

[EarthenSpire]

Those aren't chests. Those are the objects that summons the demons. His chest-crafting animation takes much longer than normal and leaves behind nothing on Zangetsu mode.