• krīḍayā — playfully
  Cousin of līlayā — pure play, no necessity. Kṛṣṇa falls not because He must but because the līlā requires the elephant's tusks to drive into the ground (and presently to be broken off). The Gauḍīyas read every Bhāgavata 'fall' of Kṛṣṇa as krīḍā, never weakness.
• dantābhyām — with his tusks
  Dual instrumental — both tusks at once. They will, in the next verses, become Kṛṣṇa's and Balarāma's weapons (43.16). The demon's own weapons turn against him: a recurring asura-fate from Hiraṇyakaśipu's nails-and-twilight (SB 7.8) to Bāṇa's thousand arms.
• kruddhaḥ — angry
  Anger is what makes the elephant strike the earth instead of the Lord. Krodha blinds — Gītā 2.63 makes the chain explicit (krodhāt sammohaḥ): rage destroys discrimination, and the demon misses by the width of Bhagavān's whole body.
• kṣitim — the earth
  Bhū-devī receives the blow. Bhāgavata theology: the earth, who in Canto 1 weeps under adharma's burden, here absorbs the demon's stroke and waits for her relief — which is precisely what this whole avatāra is.

• Where do you avoid appearing weak when strategic weakness might end the conflict?
• Could a moment of yielding draw the enemy's force into the dirt?

Kṛṣṇa falls and instantly rises — and the elephant's tusks strike not Him but the earth. The demon's force, aimed at Bhagavān, lands on the earth Bhagavān has come to protect, and even that strike is absorbed harmlessly because the Lord is the very dhārā of Bhū-devī.