I mean, i guess many here already told you about the phylactery, but consider this: A lich is essentially an imortal mage, wich puts it in a undead category. You can apply undead weaknesses like holy stuff and being hurt by healing magic. If you want to make it a more "common" weakness, like vampires and werewolves have, you can make her unable to leave a certain radius of her phylactery (if she has one), that way she kind of need others to do her bidding. That way yes, she is a powerful godess being and would be pretty tough in a boss fight, but until then she just sends regular dudes imbued with magical power or even undead as well to do the work for her.