أَشِدَّاءُ عَلَى الْكُفَّارِ رُحَمَاءُ بَيْنَهُمْ
They are firm against the disbelievers and merciful among themselves.
Ashiddāʾ expresses severity or firmness, and ruḥamāʾ mercy. Mercy is linked with gentleness, which contrasts with severity; the relationship is therefore an indirect rather than strict dictionary opposition.
The two attributes are also directed toward different groups. Strong opposition to adversaries and mutual mercy need not contradict one another because the situations are not identical.