Please help, I put myself in a corner.
So here goes. As my story progresses, it will be revealed that the main antagonist is a dead soul who was once a fire magic user. she was once a powerful magic user but in her time, women are oppressed so she was also belittled even though she was reaaally powerful. When women tried to fight for their rights, she was one of the people who openly protected those women. in the end she died. to appease the souls of those women who died fighting for their rights, the then king gave their families money to compensate for their deaths.
Main Antagonist (MV)'s family was given a large sum of money that can live luxuriously for generations and a big house. mainly because she was a great fire magic user when she was alive so they kinda took her accomplishments in consideration.
Years past and where now in the timeline of the story. There will be villains that the MC will face and win against but when she thought everything was over, MV strikes. She seeks revenge because first, her descendants are living miserably (shabby house and poor health). she thinks that the royal family neglected her family and the promise of taking care of her family was only fulfilled in the beginning and was just all talk. but in reality, her family were gambling addicts and are the ones who misappropriated their money. Second, she thinks that it is unfair that she suffered her whole life while the princess was spoiled and pampered even though she is useless. everything mainly roots from her hatred and jealousy.
How can I kill a dead soul who doesn't have a physical body nor something that can kill her as she is already dead?
First you would have to explain how a ghost without any physical form could endanger anyone. The best she could do was frighten folk. If however she is able to transcend the etheria then maybe she has a mortal core that can be trapped. So there would need to be a counter to her powers and this may simply involve a way of inverting reality and casting the hero of your story into her world. Or other way around. They meet in fires of burning red embers where all around is black and white. And the core of her power is extracted as she surrenders to the warmth of his passion.... The twist is that the hero can never return and is cast into the nether by his actions as his loved ones watch as he is consumed by fire. The family stand on the hill gazing on as the house burns to the ground from the ensuing flames. Or something like that.