Actually, Cassie is better than Batman. I mean, Shiva is better than Batman, so if Cassie can beat her, it stnds to reason she can beat Batman. Of course, the only time they ever fought it turned out to be a stand still, but I always thought she didn't try very hard since all she was trying to do at the time was prove a point and score some cuddles from him later.
I get your point that Harley may have an advantage by being slightly meta, but Cassie has gone up against metas earlier. Plus, it's hard to be unpredictable when your body telegraphs your every move a split second before you make it. (Well, it's supposed to, anyway.)