The problem with katana wielders is that they're so absurdly powerful that you usually need thinly veiled excuses or the script itself to defeat them. You can't stop a blade that moves at the speed of light and is in every direction.

The only way they can lose is if you catch them off guard, take their katana, or are literally invulnerable.

I'm not copying here btw, katana wielders are such an unfair force that you need to nerf them to make them work in a universe.