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what should i do with it? how can i beat her?
You must go to the Town Square and speak with the Town Crier to start the Quetzalcoatl questline. (Before that, you can't even fight her.) From there you will then need to get the info about the Cottage Witch, who offers you three potential ways to get the gem to defeat the Boss.
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How the hell do I beat her sister?
Persephone is a powerful combat encounter, she has claws that can't be disarmed and powerful high damage attacks. Your best bet is a mix of Stonewall, Parry + EnGarde & Slide + Uppercut.
Helena will drain your stamina with Hand Jobs, you must combo Full Nelson + Judo Toss & Low Attack to stack damage and maintain a stun-lock on her.
Together is a massive fight, you'll need to keep them both winded & Bleed Stacked otherwise Helena will grapple you and Persephone will deal massive damage. You should bring Trudy & Kylira to proc a Corn Hole Interrupt. The reward for defeating the Tag Team is just another variant of getting sandwich fucked, so your best bet is simply Dominating Helena or refusing the Tag Team Fight.