Srimad Bhagavad-Gita :
Slokam-34. ( "The Lord said: All the great warriors-Drona, Bhishma, Jayadratha, Karna-are already destroyed. Simply fight, and you will vanquish your enemies."
Every plan is made by the Supreme Personality of Godhead, but He is so kind and merciful to His devotees that He wants to give the credit to His devotees who carry out His plan according to His desire. Life should therefore move in such a way that everyone acts at Lord's will and understands the Supreme Personality of Godhead through the medium of a spiritual master. The plans of the Supreme Personality of Godhead are understood by His mercy, and the plans of the devotees are as good as His plans. One should follow such plans and be victorious in the struggle for existence.)
dronam ca bhīshmam ca jayadratham ca
karnam tathanyanapi yodhaviran,
maya hatamstvam jahi ma vyathishttah
yudhyasva jetasi rane sapatnan.
maya hatan dronam ca bhīshmam ca = Died by Me : Drona and Bhishma;
jayadratham karnam ca = jayadrathan and karnan;
tatha anyan yodhaviran = similarly other warriors;
tvam jahi = you go and kill;
ma vyathishttah yudhyasva = do not get disturbed and feel sad, start the war;
rane sapatnan jetasi = you will just conquer the enemies.
Arjuna may have doubt reflecting that Drona received the divine science of weapons by mantra from avatara or incarnation Parasurama Himself.
Also he is hesitant to slay Drona who had taught him so well the science of archery making him the greatest archer in the world.
Bhisma has received the boon that he can die at the time of his choosing and educated by Brihaspati the guru of the demigods, he was invincible.
Jayadratha has the boon from his father that whoever takes his head off at the neck with any weapon will have their own head burst into pieces.
Karna the son of the Surya the sungod has received a boon from Indra in the form of a weapon that will slay whoever he launches it at.
Also there are other warriors such as Kripacarya and Asvatthama who are undefeatable in battle.
Arjuna is certainly not doubting the words of Lord Krishna but with such a powerful array of mighty warriors against him, he was wondering how he will be able to defeat them all.
Whoever severs Jayadratha's head from his body causing it to fall on the ground will instantly have their own head burst into pieces.
Thus one who slays him is also in the mouth of death and that is why his name is specially mentioned as this was a boon given to him by his father.
Karna has special power given to him by a weapon from Indra that will slay any one it strikes but only can be launched once, Bhishma has the boon that he will die only at the time he so desires and Drona cannot be defeated by any weapon receiving the boon of knowledge of every weapon and how to neutralise them from the avatar or incarnation Parasurama Himself.
But by the express desire of the Supreme Lord Krishna they all will certainly perish on the battlefield and Arjuna and the Pandavas will be victorious.
Lord Krishna confirms that Drona, Bhishma, Karna along with innumerable warriors have already been slain by Him so Arjuna should not hesitate or feel remorse for slaying his enemies be they superiors, preceptors, relatives or friends in battle for they fighting for the unrighteous Duryodhana have already been marked for destruction by Lord Krishna.
In this slokam Arjuna is assured of victory over his enemies on the battlefield and he should not feel even the slightest doubt about this or think he is performing sins by slaying his enemies.
Lord Krishna is annihilating any doubts that may linger in Arjuna regarding chapter two slokam-s five and six when he questioned fighting superiors with the word gariyo meaning is it beneficial.
In the same way Arjuna by the code of royal kshatriyas or warriors should annihilate all opponents on the battlefield and not be distressed or afraid of any retribution as they have already been slain by Lord Krishna specifically naming Drona, Bhishma, Jayadratha and Karna.
To be continued ..