Tips for the final boss [Spoilers]
I've finally beaten the Heir, after reading a lot of tips, here are my build, and how i did it. (On Switch)
Setting of the controller: X, Doll. Y, Sword. B, Fire wand
Relics: Wine glass, leaves, bracelet and wings-looking thing
Stats: 5-5-4-7-3-4
Fight: If you previously died, save that previous spirit to have a free stun when needed. Try to dodge as much as possible, and pay close attention to the heir, look where takes its sword, if you see it point downwards, its gonna do the expanding circles, if points upwards, its going to throw what i call "rain". If the three circles appears, JUMP INSIDE THE CLOSER ONE and run as far as possible to avoid overlapping.
I've managed to get it into a stunlock when you walk towards Heir, spamming attack, it will try to avoid you, but when you reach the edge of the platform, it will lock trying to dodge, there, you can hit A LOT for free.
-Phase 2-
Right after you gain control, USE A DOLL, remember this new phase, have very ugly attacks that ruins your lifebar, so attack only when you're sure on your hits and RUN when you feel you can get hit.
If your first doll gets killed, use the next one. I had to use 3, keep going.
You'll see the battlefield getting pretty nasty, so now, you're probably at 1/2 health of the heir, now, its the endgame: Spam the crap out of the firewand, if you played like me, you'll have 28 shots, enough. Heir didn't played fair, so you're free to do the same.
Keep distance, and shoot it, i'll assure you, you got it. But RNG is a thing, so don't put much pressure if you can't do it at first time. My best tip is to set the first checkpoint you found at the start of your adventure, "village". There, you have a few enemies to build up 150 coins, and you have a shop nearby, so you can buy your dolls faster and safer.
Do not try to make a good amount of money, clear it as soon as you can so you won't lose 20 if you die on your attempts.
Good luck, and have a great fight