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This page has a list of all the people the nightmare spirits are based on as well as a brief description of that person. You can use google translate to translate the page. There's also information on what other characters are based on, as well as references in place's names.
The most troublesome part of the fight is her last life. At that point, you should do everything you can to protect yourself. Providence is a must have if you aren't already using resurrectionist ring. Onimusha has 100% crit chance so removing crits reduces her damage by 2/3rd. After that, using the buff from +10 knight sword will cut most of her damage in half again and then defending with soul shield should drastically reduce it even further. You can also summon meat wall if you have it so there's a chance she will attack him instead of you.
After that is Mabel herself. She doesn't attack you but every few turns she summons 1 of 3 bosses (sleeping god, lake god, and black goat) until they are all summoned. Once you defeat those 3, you automatically win. However, you can also win just by defeating Mabel, which I recommend as she does not have significantly more health, you just have to watch out for her counter attack which she will announce when she is using it. With max attack, you should be able to kill her in 2-3 turns before she can even summon.
Go full glass cannon if you have to, 100% crit (hornet ring+3, reckless hero ring, king's order, onimusha armor/helm), celestial ring. You want around 20-25k attack before going into the fight to reach max attack with buffs. The rest of your gear should be focused bonus turns like andor sword (just the start of the fight or if you are still buffing), black rabbit rings, duchess coven (make sure you still have over 5k speed). Buffs include attack up buff(king's command preferably since it only takes 1 action), insanity (jub jub bow or golden mead), reinforce (from vorpal blade or manually with the pine resins. If you use resins, make sure you have frigid resin.) and phalanx. Debuff her with sin burst from black blade, frozen magic bullets to reduce her ice resistance, then destroy her with hans. Alternatively to frozen magic bullets, if you equip banker's ring, you can oil her at the same time you use hans basic attack but this takes up a ring slot in exchange for an action. With this set up, I can turn 1 her easily.
Fear ring is from a nightmare spirit in ludwidge. He's south of the gallows in a small courtyard. The gallows are southeast of the bonfire.
