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.

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.