Just threw her into an opening scene to get things rolling. But then, with hindsight, realized that meant she had to tie into the end.
Mind you, the heroine doesn't have to deal with her. I could have another villain deal with her, and then make that villain the final problem. Of course, that means that villain has to fight her, since half the point would be to beef up that villain into a great challenge.
At least I had her be a crude, unsophisticated sort of person. She'd turn a wall into nitrogen, or a car into lead, but nothing so refined as turning all the oxygen in your lungs into chlorine.
First thought was force fields. Or something else equally non-material and pure energy. Illusions, perhaps? Mental powers, control and the like, are right out because I've excluded them from the universe. Surprise would, I think, work only if another villain took her down.