• katthase — you boast
  Middle voice of √katth — to brag. Used in Mahābhārata for the empty speech of those about to fall. Bhāgavatam imports the same usage: katthana is the demon's reliable predictor of his own immediate death.
• antakam — death
  He who ends — Yama personified. Kṛṣṇa as Antaka stands literally beside Śālva at this moment. Gauḍīyas note that Bhagavān names His own role plainly when speaking to demons; with devotees, the same Lord conceals it under a flute.
• śūrāḥ — heroes
  From √śūr — to be brave. The class of warriors who act rather than boast. The verse's pivot — Kṛṣṇa, who is Himself Śauri (descendant of Śūra), defines what a true śūra is, in the very moment of being one.
• pauruṣam — prowess
  From puruṣa — manliness, the strength of being. Gauḍīyas hear a deeper register: pauruṣa is what the puruṣottama displays. Real strength is shown by Bhagavān, not by demons who only describe their own.

• Where do I substitute talk for action?
• What single action this week would do more than all my words?

"You boast in vain; you cannot see death standing beside you. Real heroes do not talk much; they show their prowess in action." Kṛṣṇa's reply is the entire sūtra of bhakti's stance toward boasting demons — one sentence, total composure, no rhetorical match for the demon's catalog.