अन्वयः

ब्रह्मदत्तवरः by the boon given by Brahma, एषः in that way, अवध्यकवचावृतः covered by armour which can't be broken, एनम् therefore, ब्राह्मेणअस्त्रेण by Brahma's weapon, भिधनि strike, एषः because, अन्यथा other, वध्यः to kill, न not, कवची armour, बली strong, एषः in that way, अन्येषाम् with any other, अस्त्राणाम् by other weapon, अवध्यः not possible to kill.

M N Dutt

Then approaching him, the wind-god spoke to him, saying, "This one has received a boon from Brahmā, and is covered with an impenetrable mail. Pierce him with a Brahma* weapon. He cannot otherwise be slain. This one clad in mail and endowed with prowess is forsooth incapable of being slain by others." *Belonging to Brahma.

Summary

By the boon given by Brahma his body is covered with armour which can't be broken. By Brahma's weapon only you can strike to kill him because of his armour and might. By any other weapon it is not possible to kill him.

पदच्छेदः

ब्रह्मदत्तवरो	ब्रह्मन्–दत्त (√दा + क्त)–वर (१.१)
ह्येष	हि (अव्ययः)–एतद् (१.१)
अवध्यकवचावृतः	अवध्य–कवच–आवृत (√आ-वृ + क्त, १.१)
ब्राह्मेणास्त्रेण	ब्राह्म (३.१)–अस्त्र (३.१)
भिन्द्ध्येनम्	भिन्द्धि (√भिद् लोट् म.पु. )–एनद् (२.१)
एष	एतद् (१.१)
वध्यो	वध्य (√वध् + कृत्, १.१)
हि	हि (अव्ययः)
नान्यथा	न (अव्ययः)–अन्यथा (अव्ययः)

छन्दः

अनुष्टुप् [८]

छन्दोविश्लेषणम्

१	२	३	४	५	६	७	८
ब्र	ह्म	द	त्त	व	रो	ह्ये	ष
अ	व	ध्य	क	व	चा	वृ	तः
ब्रा	ह्मे	णा	स्त्रे	ण	भि	न्ध्ये	न
मे	ष	व	ध्यो	हि	ना	न्य	था