بِأَحْكَمِ الحَاكِمِيْنَ

بِأَحْكَمِ الحَاكِمِيْنَ is a phrase (شِبْهُ الجُمْلَةِ), a genitive particle followed by a possessed noun and a possessive noun

PhraseParticle / Noun / NounNounNoun Particle
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌ وَهُوَ مُضَافٌ وَ مُضَافٌ إِلَيْهِمُضَافٌ إِلَيْهِاسْمٌ مَجْرُوْرٌ وَهُوَ مَضَافٌحَرْفُ جَرٍّ
PhraseCompoundPossessive NounGenitive noun + Noun possessedParticle of pulling down
  
the most just of the judgesبِأَحْكَمِ الحَاكِمِيْنَ=الحَاكِمِيْنَ+أَحْكَمِ+بِ
the most just of the justicesthe justicesthe most just of
Surah/Below is the only verse in the holy Quran containing the genitive particle and nominal compoundبِأَحْكَمِ الحَاكِمِيْنَ
Verse
97-8Is not Allah the most just of all judges?أَلَيْسَ اللهُ بِأَحْكَمِ الْحَاكِمِيْنَ