Aw sad to see the Onimusha franchise losing activity like this.
But yeah for ten point slash I first got it by just closing my eyes once I heard the issen flash, then pressed it at random times. There's like a half second extra gap between each button press. Once you get the rhythm it is so cool to see. Esp in the final fight.
Btw, the 2nd Oni game to me has the best replayability. Always something new to notice.