Bhagavad Gita 11.34द्रोणं च भीष्मं च जयद्रथं च कर्णं तथाऽन्यानपि योधवीरान्।मया हतांस्त्वं जहि मा व्यथिष्ठा युध्यस्व जेतासि रणे सपत्नान् ॥

Already slain by Me Therefore, O Arjuna, you need not be afraid of incurring sin by killing them.Drona had celestial weapons. He was Arjuna's teacher in the science of archery. He was Arjuna's beloved and greatest Guru. Bhishma also possessed celetial weapons. He could die only if and when he wanted to die. He once faught singlehanded with Lord Parasurama and was not defeated. So powerful was he.The father of Jayadratha performed penance with a resolve the head of the man who may cause the head of my son to fall on the ground, shall also fall. During the war, however, Arjuna's arrow cuts the head and drops it on the lap of the father who, inadvertently, makes it fall on the ground he too dies at once. Karna, the son of the Sungod, had obtained an unerring missile from Indra.