• śilā-varṣam — rain of stones
  Compound: śilā (stone) + varṣa (rain, shower). Echoes Indra's stone-and-water deluge in the Govardhana līlā (SB 10.25); there Kṛṣṇa lifted the hill, here Balarāma pulverizes the stones. Brothers, parallel līlās.
• amarṣitaḥ — frustrated, unable to bear
  From a-marṣa (intolerance). Asura-prakṛti cannot bear being unsuccessful; the inability to endure setback is itself a diagnostic. Compare the bhakta's titikṣā (forbearance), which is the same situation seen with bhakti.
• cūrṇayām āsa — He pulverized
  Periphrastic perfect of caus. √cūrṇ (to grind to powder). The same verb the demon used in v.4 against kingdoms (samacūrṇayat) — the Lord returns the asura's own verb to him. Karmic poetry.
• līlayā — easily, as play
  Instrumental of līlā. The technical term. What is mahā-prayatna for the asura is līlā for Bhagavān. Gauḍīyas insist all His acts — even battle — are līlā, not effort.
• muṣala-āyudhaḥ — the wielder of the club
  Compound: muṣala (club) + āyudha (weapon). His standing epithet; weapon and wielder are one. The asura's weapons are improvised (trees, stones); Bhagavān's are eternal aṅgas of His vigraha.

• What 'stones' have I been throwing at God's plan that He has been patiently deflecting?
• Where could I stop throwing and start listening?

When trees run out, the demon turns to stones — pralaya in miniature, hail upon the Lord. And the Lord pulverizes the storm līlayā, "easily, in play." What asuric effort costs the world dearly, Bhagavān undoes as sport.