Relative pronouns are one of the biggest hurdles for middle school students learning English.
Literal
Middle-school-students [subject-が] English [object-を] learn [time-際][possessive-の] greatest-difficulty [possessive-の] one [subject-が] relative-pronouns [copula-です].
最難関 'greatest difficulty/hurdle' — 最 'most' + 難関 'difficult barrier.' 際 'at the time of, when' is a formal temporal marker. The sentence is from the English education context, noting a well-known stumbling block for Japanese learners.