Stelle can be irritating because she is so fast, and she dodges alot. However, there is one thing she can't do very well, jump.
If you are struggling with steele, run to the back right of the arena from where you start, there are a couple of white blocks, standing on these will leave you pretty much invulnerable to most of her attacks. Just keep shooting, allowing her time to stand still seems to give her a chance to aim her ranged attack at you better. I beat her on level 61 in playthrough 2.5 and only got hit twice.