Complex sentence: Kōsei is formerly a child prodigy in playing piano, often dubbed as the "Human Metronome", he was able to play the piano with extreme precision and discipline and thus had won many competitions across Japan.
Split into simpler sentences: Kōsei is formerly a child prodigy in playing piano, often dubbed as the "Human Metronome". He was able to play the piano with extreme precision and discipline and thus had won many competitions across Japan.