समाहिताः settled, निपतन्तः fallen, शिखिसम्पर्शाः touching, ते those, शराः arrows, वज्राणि thunderbolts, महागिरीन् इव like mountain, राक्षसान् Rakshasas, दारयामासुः split

Those burning arrows falling on Rakshasas touched the Rakshasas and split them like thunderbolt splitting mountains.