«أَشِدَّاءُ عَلَى الْكُفَّارِ رُحَمَاءُ بَيْنَهُمْ»
“Firm against disbelievers, compassionate among themselves.”
Ashiddāʾ and ruḥamāʾ contrast severity and mercy under different relations. The rhetorical effect is not merely a mechanically paired dictionary antonym, but opposed conduct in context.
Not every pair of qualities constitutes ṭibāq. There must be a relevant contrast that contributes to the meaning.