• mahā-vīryaḥ — of great potency
  Compound: mahā (great) + vīrya (heroic power). Echo of v.2's vīrya-vān. The chapter keeps reasserting the demon's stature so the reader cannot dismiss Balarāma's victory as easy. The size of the asura is the measure of His mercy.
• śālam-udyamya — lifting up a śāla tree
  Śāla — a great sal tree, classical emblem of forest-strength; the same trees Rāma shoots through with one arrow in the Vāli-episode. Vānara-vs-śāla imagery is Rāmāyaṇa-fed; SB invokes it deliberately as backdrop to Balarāma's victory.
• tarasā — swiftly, with force
  Instrumental of taras (speed, energy). Force without bhakti — the asura's only mode. Compare the same word in bhakti-mode: tarasā tartum eva = swiftly cross over (the ocean of birth-death) by surrender.
• mūrdhani-atāḍayat — he struck on the head
  Locative mūrdhan (head) + imperfect of √taḍ (to strike). To strike Bhagavān on the head is the maximal offense, dramatically; the verse holds that blow up so v.18's acalaḥ-yathā response can stand against it as immovability.
• abhyetya — approaching, closing in
  Abhi- + √i — to come up to, to confront. The asura at last comes within striking range of Bhagavān; the geometry the whole chapter has been narrowing finally closes.

• Tamohara Pr SB 10-67-17-23 - 2015-06-16 ISKCON Alachua
  His Grace Tamohara Prabhu· 2015-06-16· English

A śāla tree, hundreds of years in the growing, becomes a club in the asura's fist within a single breath. Demonic strength borrows everything — even the patient gifts of the forest — and uses them only to strike at Bhagavān.