Try reading this guide from the devlog, hopefully it should help. https://lj-lephemstar.itch.io/scratchin-melodii/devlog/653636/scratchin-melodii-...
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Yes I know that guide, I have read it and it still doesn't feel pleasant to play it or freestyle.
I'm not hating the game, I like the aesthetics, I like the songs, I like everything. But playing it just feels wrong. I understand that the game is inspired by parappa but it shouldn't feel just as inconsistent with the score and grades. I'm not a game developer either, maybe it's just my fault. But throughout my playthrough I kept feeling frustrated by the game taking away my score for a line that I felt I did well and I would just like to give this feedback from someone who really likes rhythm games.
From my experience, the scoring is sort of consistent. Some rare lines feel weird, like Rensa's "LEFT LEFT-LEFT" most of the times when I press all three LEFTs it gives me a negative. Instead I end up just pressing the first two and that almost always lands me a positive. It's not easy to get A and S, you have to learn the songs, try the different lines, see when you can freestyle and how, and come up with a plan to make points.