It's fun to pit characters from one company against characters from another. After all, now that Cass doesn't exist, any fight between her and another character is strictly hypothetical, so why not be the hypo-ist of the thetical? I seem to toss Cass up against heroes, though. I enjoy that more than a hero-villain dust up and the results are easier to plan or predict. A discussion I didn't bother to read on Comic Vine inspired this, the latest entry in my "Cassandra Cain versus" series.
What happens when martial artist Cass fights martial artist Iron Fist?
All things being equal, she absolutely would win with her move predictive ability and resulting defensive superiority, while matching Iron Fist's offensive skills. Perhaps she's not as physically strong as he is, but she'd make up for that with speed and endurance. She would also be learning his techniques as they fought, thus gaining all of his skills plus her own.
Unfortunately, all things aren't equal. Danny "Iron Fist" Rand has a lot of supernatural/metaphysical martial arts powers that, frankly, put him out of Cass's class, so I seriously doubt she could defeat him. Iron Fist could focus his chi and overmatch Cass in every category and she couldn't learn those things simply by observing and sparring anymore than she could learn Spider-Man's spider-sense or Mr. Fantastic's stretching ability. Cass has a chance if she somehow provoked Iron Fist into using his "iron fist" ability, somehow dodged the strike, then counter-attacked while he was drained.
Failing that, her only course would be purely defensive-- staying out of Iron Fist's reach and then escaping. Sorry, Cass. If it's any consolation, I'm pretty sure you could beat Shang-Chi, master of Kung Fu, and he's no slouch himself.