Brahma Danda

The reason why Drona proved unconquerable on the 15th day of the Mahabharata war at Kurukshetra was because he used the BRAHMADANDA for the 1st time ever in his life.

http://1vir.blogspot.com/search/label/Drona

As Drona had not imparted knowledge of this secret weapon to either Arjuna or Ashwattama, therefore no Pandava, Yadava or Panchala MAHA-RATHI/ATHI-RATHI could handle Drona on the 15th day of battle.

http://en.wikipedia.org/wiki/Brahmanda_astra 

Therefore the Pandavas were forced to do cheating to kill Drona

https://p01z.blogspot.com/search/label/WarPermit

Dhrishtadyumna mustered all his energy. Laying down on the car his formidable bow, with arrow fixed on the bow-string, he took up a sword, and jumping down from his vehicle, rushed quickly against Drona. All creatures, human beings and others, uttered exclamation of woe, beholding Drona thus brought under Dhrishtadyumna's power. Loud cries of Oh and Alas were uttered, as also those of Oh and Fie. 

As regards Drona himself, abandoning his weapons, he was then in a supremely tranquil state. Having said those words he had devoted himself to Yoga. Endued with great effulgence and possessed of high ascetic merit, he had fixed his heart on that Supreme and Ancient Being, viz., Vishnu. Bending his face slightly down, and heaving his breast forward, and closing his eyes, and resting ort the quality of goodness, and disposing his heart to contemplation, and thinking on the monosyllable Om, representing. Brahma, and remembering the puissant, supreme, and indestructible God of gods, the radiant Drona or high ascetic merit, the preceptor (of the Kurus and the Pandavas) repaired to heaven that is so difficult of being attained even by the pious. Indeed, when Drona thus proceeded to heaven it seemed to us that there were then two suns in the firmament. The whole welkin was ablaze and seemed to be one vast expanse of equal light when the sun-like Bharadwaja, of solar effulgence, disappeared. Confused sounds of joy were heard, uttered by the delighted celestials. When Drona thus repaired to the region of Brahman, Dhrishtadyumna stood, unconscious of it all, beside him. Only we five amongst men(Krishna Aswatthaman were Vishnu Shiva) beheld the high-souled Drona rapt in Yoga proceed to the highest region of blessedness. These five were 

1. myself, 
2. Dhananjaya, 
3. the son of Pritha, and 
4. Drona's son, 
5. Aswatthaman, and 
6. Vasudeva of Vrishni's race, and 
7. king Yudhishthira the just, the son of Pandu. 

Nobody else, O king, could see that glory of the wise Drona, devoted to Yoga, while passing out of the world. In fact, all human beings were unconscious of the fact that the preceptor attained to the supreme region of Brahman, a region mysterious to the very gods, and one that is the highest of all. Indeed, none of them could see the preceptor, that chastiser of foes, proceed to the region of Brahman, devoted to Yoga in the company of the foremost of Rishis, his body mangled with arrows and bathed in blood, after he had laid aside his weapons.


https://www.sacred-texts.com/hin/m07/m07189.htm

Drona's Prayer to Krishna

anaayaasena maranam / vinaa dainyena jivanam
dehi me krpayaa krishna / tvayi me bhaktim acancalaam

"I should get a sudden death without any prolonged illness (this he asks mainly because he does not want to put others into inconvenience), I should not depend upon anybody for my life's needs, and as long as my soul is in this body, I should do steady unflinching devotional service unto You."

https://www.google.com/search?q=anayasena+maranam