Comic Vine's discussion boards are a goldmine of super-wonky comic book discussions about who would win in practically any fight you can imagine. Now the closest things to superpowers Cass has are her abilities to read people's body language and therefore anticipate their next moves and to learn new fighting styles in a matter of minutes just by watching. Batman taught her stick fighting from novice to expert in about five minutes. Now since there's no way he showed her every possible strike, counterstrike and defense in those five minutes or so, we have to assume some sort of higher function when it comes to understanding martial arts-- once she grasps the basic concept, she can use her superior skills and strangely-wired mind to extrapolate from there until she has that expert-level proficiency without having to spend years in practice.
What I'm getting at is what happens when she fights someone with similar abilities? Like Marvel's Taskmaster? Taskmaster can almost instantaneously learn someone's fighting skills just by watching them, which then gives him something like Cass's predictive ability. Well, the fans at Comic Vine hash it out pretty thoroughly, but here's my take.
All things being equal, and both unarmed, I think his greater strength gives Taskmaster the edge, but if Cass can stay a step ahead of Taskmaster, she has a chance to wear him down. Taskmaster and Deathstroke are roughly analogous, and Deathstroke has always given Cass fits. Cass would have a better time predicting Taskmaster's moves than she did Deathstroke's because Deathstroke had some kind of funky kinetic dissonance going on that confused Cass, and she'd be learning from Taskmaster and gaining skills (while Taskmaster would peak out once he'd watched Cass's base martial artistry-- all else would be superfluously his own), but I don't think her greater speed takes his size and mass.
But Taskmaster isn't stupid, so he'd ambush Cass using his sword or fake Captain America shield. Surprised and off-balance from the start by Taskmaster's assault, there's no way Cass develops the skill level fast enough to win even if she found a sword of her own to use. Yes, I said Cass would beat Katana under similar circumstances, but Katana isn't Cass's match in pure martial artistry.
Unfortunately, I have to give this one to Taskmaster. It would be one heck of a fight, though.