[Machine translated]
I've tried various builds, but the combination of Bloody Sacrifice, Flying Sword, and Regeneration Ring is excellent in many ways. Its defining feature is its extreme safety, and it is powerful enough to handle any enemy.
While it requires quite a few elements, once you secure a decent number of Flying Swords and Regeneration, it seems you can quickly eliminate anything except perhaps a group of two bears.
Actually, if you cannot secure a Flying Sword +4 or higher book, you have to go with a significantly different setup as an alternative... Animating Weapon or Water Spirit are primary alternatives.
If you cannot obtain even those, then Ring of Destruction + Rod and many offensive spellbooks.
Anyway,
Flying Sword is truly versatile. Its only weakness is Poison Mist. After all, Flying Swords die instantly upon contact with Poison Mist. However, this is not a major issue. Due to sheer numbers, no enemy can even approach it.
Interestingly, plants that generate Poison Mist can be dealt with using the Flying Sword summoning ability. Previously, when summoning the Flying Sword, you could counter it by utilizing the phenomenon where it would be summoned near an enemy if one was nearby.
Now, since using Bloody Sacrifice fully recharges the spell itself so you can use it immediately, you become incredibly powerful even with just two or three decent spells. You can freely unleash powerful ranged attacks or powerful summons.
In my experience, equipping two Rings of Regeneration +3 makes this routine possible:
Bloody Sacrifice -> Powerful Attack -> Bloody Sacrifice -> Powerful Attack -> LOOP.
With two Rings of Regeneration +3, losing 20 HP every 2 turns is manageable.
Once you have a lot of attack options, you don't even need the Rings of Regeneration. You can freely use Powerful Attacks, and once the enemy is gone, repeat Bloody Sacrifice to recharge all attacks; your HP will be fully restored while you move to a safe zone free of enemies.
Ultimately, what makes it excessively overpowered is the ability to reuse Bloody Sacrifice multiple times in a short period of time.
My personal balance adjustment proposal is to reduce the performance of Bloody Sacrifice to a certain extent. HP cost 13, recharge all spellbooks +0.666.
...Of course, compared to the original, the performance feels ridiculously weak... but instead, we would allow Bloody Sacrifice to be recharged up to 2 times.