Most fantasy – at least the heroic sort – seems to be a cross between medieval and modern in outlook, with magical elements. Fortunately it’s not entirely medieval, because the medieval times were both disgusting and really quite alien in terms of the way people thought. The upshot of that is that a standard feminist (whatever the hell that means) outlook would be extremely unlikely in that kind of world. A woman wanting to run her own affairs would be up against not just men but other women and the Church, in which, being medieval, she’d almost certainly devoutly believe. So it wouldn’t just be men keeping her down, but the word of God. Although there were some female saints, they were usually chaste and suffered considerably, which few would want to emulate. That’s a lot to rebel against. (For a demonstration of medieval tolerance, see the death of Richard II).
So I’d have thought that to be credible in fantasy, a female character would have to have some sort of history to base herself on or at least ‘excuse’ her conduct. The Order of Joan of Arc, or something similar, could allow female fighters to operate. Wizardry would put an interesting spin on things, by making some women inherently very powerful, although religion being what it is they would probably be seen as something to be destroyed unless they were able to secure themselves.
And then there’s the practical problem of physique, especially when being pale and plump was seen as a sign of wealth. At the very least, I’d expect a female swordswoman to have the rather stringy, muscled physique Madonna had a few years back. I suspect most men find that unattractive: I certainly do. (That said, I did once know a girl who was about 5 feet tall, pretty and skilled in ninjutsu. But against a trained male fighter, who knows?). It’s possible, I guess, but don’t expect a lady swordsman to look that kind of fit.
Which brings us onto Mary Sue. I suspect the answer is just to accept that attractive girls who can fight in close combat are about as common as manly lone woodsmen who are actually kind to puppies, or anyone even slightly resembling Jude Law in a pre-moisturiser setting. Of course, fantasy does let writers do what they want, but it’s got to make some sort of sense. Oh, and wearing high heels in a fight is just plain stupid. I wish male artists would stop drawing them like that.