As great as this game is... the final boss fight sucks... From just one move...a healing over time move that restores around 1/7 of her hp per turn, and it's already painful to get through her normally...
you definitely need scaling damage. Dark fairy or even regular fairy have their damage buffed over time. So they can deal the big numbers and she can’t out heal it.
A.E.R.A from the machine pack also has that option.