The upper slash move? I can reduce the amount the enemy knocks back to make it easier and increase the range more but I just tested it on this web version. Do you have a video so I can see the issue?
If it helps for investigating the problem, I just turned on a framerate monitor to check. And it was a solid 239 with occasional steps up to 240 (I have a 240Hz monitor so that's normal for me to see in games which don't have an arbitrary lower fps cap).