ततः then, सः he, उत्तमम् best, महावेगम् endowed with impelling force, महानादम् huge sound, धनुःbow, मेदिनीम् earth, निर्भिन्दन्निव as if breaking, विस्फारयितुम् began to stretch, आरेभे started

Then he began to stretch his best bow endowed with an impelling force, sounding as if breaking the earth.