• mahī — the earth
  Mahī — the earth, also bhū-devī. The same earth-goddess prayed to Brahmā in SB 10.1 for relief from demonic kings; Kaṁsa unknowingly names the very being whose petition started the whole avatāra-mission.
• mitra — O friend
  Address to Akrūra. Kaṁsa uses friendship-vocabulary; for Gauḍīyas the irony lands hard — true friendship is Kṛṣṇa-sakhya, which Akrūra is about to enter; Kaṁsa's mitra-bhāva is the simulacrum.
• naṣṭa - kaṇṭakā — free of thorns
  Naṣṭa (destroyed) + kaṇṭaka (thorn). Bhāgavatam's stock idiom for a peaceful realm. Kaṁsa imagines the thorns are Kṛṣṇa-Balarāma-Ugrasena-Vasudeva — exactly inverted from reality, where the thorn is himself.

• What 'thornless world' do you secretly imagine that depends on others being removed?
• Where might the real thorn actually be inside your own attitudes?

'Then the earth will be free of thorns.' One half-line — the asura's vision of utopia. The Bhāgavatam pairs his line with its silent rebuttal: the earth is freed of thorns precisely when Bhagavān removes Kaṁsa Himself, six chapters from now.