«مَنْ يُنَقِّحُ الْإِنْشَاءَ وَيَتَصَرَّفُ فِيهِ كَيْفَ شَاءَ»
Someone who refines prose and can shape it at will.
Yunaqqiḥu is a present-tense Form II verb, associated here with editing and polishing a composition. Yataṣarrafu is a Form V verb: the writer handles expression with flexibility and control.
The two abilities complement one another. Refinement removes weaknesses; flexibility allows the writer to adapt to subject, audience, and purpose. The assembly doubts that anyone living can still possess such mastery.