They are blazing fast at recovering from blocking.
So how do You defeat them?
(For me they are either attack allways or do nothing as they will kill me anyway, categories and nothing in the middle)
This is a long post, but I've kinda wanted to write about it, so here it is. :P
Sword duels are a bit wonky. Practice makes perfect for sure, but here's the things I've learned. Most of this stuff is theoretical. It's hard to know for sure which things are true.
The way I typically win fights is by staying just ouside their attack range on the left or right, doing a horizontal slash as they approach just close enough. You have to swing in one fast motion. Holding your sword out for more than a second, hoping they'll walk into its path, is usually met with a blank stare by the enemy.
Where you're standing seems to affect how often you hit, especially if you have them pushed back up right against the wall. Sometimes I'll stand right in front of them doing forward slashes non-stop, and it can work then.
You can use Backspace to block, (maybe there's a more convenient key). I think if you do this, they're less likely to block a counter attack, (by that I just mean attacking quickly afterward). The problem is, I can't seem to get the hang of it.
Doing a full slash, where you hold attack and down, followed by a forward strike, is unblockable. You have to be lucky to be able to pull it off without them moving out of the way, but it works like a charm if you can get them in front of you.
Sometimes you can dodge a swing, and I think that makes a counter attack more likely to hit, too.
This might sound weird, but I think if you walk left and right, kind of like head faking, from their left to right side a couple times, it throws them off, making them more vulnerable. You just have to not get slashed.
Anyway, sometimes you just have to suffer them blocking a lot, especially on Katana or higher difficulty. Especially if your swordplay is bad and theirs isn't.
The last thing I'll mention is that if you use the numpad, you have a lot more options. Like if you hold attack and press 7, it does a forward-and-to-the-left slash. No idea if this actually affects gameplay or if it just looks different.
Hopefully some of this helps.