You have to jump when you're doing it, too. if you touch the ground before it finishes, it won't work and you'll just get shocked.

If you clear some post-game optional challenges, there's a special move you can unlock that does it easier, but that's only useful if you want to do NG+

Hours on the centipede man, you say? Once you get into the deflect rhythm, they're very satisfying. Fill their posture right up and then finish them. But without the timing, you're going to take a lot of chip damage or get posture broken yourself :(

I think Juzou and Genichiro is where I stopped. Game's offense is so boring I realized I'll have much more fun if I replay Ninja Gaiden 2 again, so I went and did that instead.