difficult piece of music
Refers to a musical composition that is technically challenging to perform. Common in classical and instrumental contexts.
This piano sonata is a really difficult piece.
彼は難曲をさらさらと弾きこなした。
He played the difficult piece effortlessly.
Passion music
Compound of 難 (なん, 'difficult') and 曲 (きょく, 'musical piece').