Only barely counts as a boss battle. Just pick up the girders and press B to throw them at her. It only takes a few. It's been a few years but I believe harder difficulties require more girders.
She's not too hard to deal with because she is mostly just running away from you. Be sure to dodge or block her occasional projectile attack.
Poor Viola. If you saw Idolo and how she started out, seeing her fate as a Mad AI is a bit sad.
Please leave a comment if you decide to vote down this solution.