• saṁrabdhaḥ — furious
  sam + rabdha — fully seized (by anger). Gauḍīya: saṁrabdha is a strong rage-word, used elsewhere for Hiraṇyakaśipu and Rāvaṇa. It signals the demon's last stand — fury preceding kill.
• upakroṣṭā — the ass
  upa-kroṣṭṛ — literally one who brays nearby; an alternate name for ass. Gauḍīya: the Bhāgavatam uses synonyms for ass repeatedly — rāsabha, kroṣṭṛ, khara — perhaps to underscore how thoroughly Dhenuka's form is what he is.
• parāk sthitaḥ — standing with his back toward the Lord
  parāk (turned away) + sthita (standing). The position of insolence. Gauḍīya: turning the back to Bhagavān is the very definition of saṁsāra in CC Madhya 20.117 — kṛṣṇa bhuli sei jīva anādi-bahirmukha. Dhenuka enacts it physically.
• caraṇau aparau — hind legs
  caraṇa (foot) + apara (hind). Dual. The two hind legs. Gauḍīya: the rear-kick is asura-mode; the front-bow is bhakta-mode. The same limbs, differently directed, define disposition.
• ruṣā — with anger
  Instrumental of ruṣ (anger). Gauḍīya: ruṣa, unlike saṁrabdha, is the act-tense of anger — the swinging blow. The demon's hind legs are tools of his ruṣa; Balarāma's hands are about to be tools of liberation.

• What recent attack on me felt loud but actually had no real substance?
• Where am I confusing volume with strength in something opposing me?

An ass turns its back to a king and kicks — the lowest form of attack, possible only to the lowest creature. Dhenuka, furious, presents his rear to Balarāma and hurls his hind legs. The verse documents the precise insolence the līlā will answer. Sometimes the Lord's response is calibrated to the offense's exact shape.