• nirudhya — besieging
  Gerund of ni + √rudh (to obstruct). The same root behind nirodha — yogic restraint. The asura's nirodha is a siege; the yogī's is of the senses. Same grammar, opposite directions.
• vimāna-agryāt — from the best of airships
  Contiguous join (vimāna + agryāt). Vimāna is the Vedic flying chariot — devas have them, asuras imitate. Agrya means foremost; even the foremost asuric craft is, in Bhāgavata measure, less than a devotee's bare feet on Vraja's dust.
• śastra-vṛṣṭayaḥ — torrents of weapons
  Contiguous join (śastra + vṛṣṭayaḥ). Vṛṣṭi literally means rain — the asura's offering to Dvārakā is a perverse rain, mirroring the prema-vṛṣṭi that devotees pray Kṛṣṇa will rain upon them (cf. Caitanya-caritāmṛta's prema-megha imagery).
• cakravātaḥ — a whirlwind
  From cakra + vāta. A cyclone. Prabhupāda's commentary reads Śālva's whirlwind as māyic show-business — calculated terror against a city protected by the Lord's own sudarśana-cakra. Cakravāta against cakra-pāṇi: the wind cannot win.
• purīm — the city
  Accusative of purī. The same word used for Vaikuṇṭha and for Kṛṣṇa's heart. The Bhāgavatam often reads attack on the Lord's city as the asura's confession that the Lord has a city — that He is personal, located, available.
• rajasā — with dust
  Instrumental of rajas. Pun: rajas the substance (dust) and rajas the guṇa (passion). The whole asuric assault is rajas-blanketed — covering the directions, the way passion covers the cit-svarūpa of the jīva.

Like hailstorm and locust together falling on a garden the King has built for His queens, Śālva's downpour from Saubha tries to ruin Dvārakā — yet the Lord of the garden has not yet stirred; what falls is the asura's whole arsenal at once, a single throw of every die he owns.