While I think it's a little problematic Scott Peterson and Kelley Puckett made the decision to give Cass language so early in her solo series, there's no denying this led to some of its hardest-hitting moments, both figuratively and literally.
The change came with a price. Because her upbringing altered her brain's architecture in unexpected ways, in order for Cass to learn language a psychic had to reconfigure her mind. This cost her a large part of her fighting abilities. She could no longer read body movement as if it were language, which mean she couldn't predict her opponents' moves. If I'm reading Cass's background correctly, because she had this almost pre-cognitive ability her father-trainer Cain didn't see fit to instruct her in practical defense. He thought it best just to shoot her occasionally to "keep her on her toes."
The point is, gaining language complicated Cass's life and made her vulnerable in ways she hadn't been before. To fix things, she turned to Lady Shiva. Lady Shiva wasn't about to give her secrets away for free, so she and Cass made a deal. One year of perfection, then a death duel. This played into Cass's death wish and also created a lot of suspense for twelve or so issues, until the two finally tangled.
That took place in Batgirl #25 (April 2002), one of the high points of the entire series. The fight displays Damion Scott's action sequence storytelling prowess. It's six pages of kung fu madness with the two characters trading vicious blows. Cass shatters a sword with her hand, Lady Shiva elbows Cass in the belly, Cass returns the favor, Lady Shiva punches Cass squarely in the face. Scott draws it with just about every technique you can imagine-- silhouettes, stacked panels, a collage-like page, insert panels. It's some bravura stuff.
But you don't have to take my word(s) for it. Check out Friday Night Fights over at Hero Sandwich. All six pages! Let's count them! One beautiful page of Cass fighting Lady Shiva! Two beautiful pages of Cass fighting Lady Shiva! THREE beautiful pages of Cass fighting Lady Shiva! FOUR BEAUTIFUL PAGES of Cass fighting Lady Shiva! FIVE! FIVE BEAUTIFUL PAGES OF CASS FIGHTING LADY SHIVA! SIX! SIX! SIX BEAUTIFUL PAGES OF CASS FIGHTING LADY SHIVA! AH HAHAHAHAHA!
(Cue thunder and lightning)
I don't think it's spoiling things to mention Cass survives the fight. After all, her title ran for about forty-eight more issues and she went on to that whole heel-turn controversy, redemption and a new identity before being written out of continuity for however long DC sees fit.