Drona, Bhishma, Jayadratha and Karna, as well as other brave warriors, them – already killed by Me – do you kill and be not distressed. Fight, and you will conquer your enemies in battle. (Bhagavad Gita Chapter XI – Verse 34)
There was good reason for Arjuna to fear his opponents. For some of them were endowed with special powers and celestial weapons and others were considered invincible.
Bhishma, for example, had his death at his own will. He had his life under his own control, for there were only certain conditions under which he could be killed. By avoiding these conditions he was safe from all attack.
Karna was most formidable in war and Indra, being pleased with him, had given him an unerring missile. That missile was sure to kill the person at whom it was directed.
Jayadratha’s father, again, was performing severe austerities in order that whoever killed his son would himself die instantly. And so were there others.
Besides, Arjuna had reason to fear that he would commit a great sin by killing Drona and Bhishma. The latter was his grand sire who had always been very kind to him and the former, his greatest teacher in the science of archery. It was Drona who had taught Arjuna all that he knew, who had made him the greatest of all warriors. It is for this reason that Sri Krishna especially mentions these heroes when he tells Arjuna not to feel troubled. Victory will be yours and you will not commit sin by engaging in the fight. Some way or other these men will lose their lives. I have ordained it so.
Sanjaya again relates to King Dhritarashtra the effect which Sri Krishna’s words exercised on Arjuna.