Again, Cass had trouble beating the Joker in a fight. Harley's got the Joker's advantage, plus the fighting skills, enhanced strength, speed, and reflexes. Cass has the advantage in fighting skills, but Harley is just a little bit stronger, a little bit faster, and reacts a little bit quicker than Cass.
I'd guess she'd also have problems with Deadpool and this guy who was one of Harley's teammates in the Justice League of Anarchy...