Why did Dronacharya prove unconquerable on the fifteenth day of the Mahabharata war ?

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. Brahmadanda was Brahmarishi Vashistha's best weapon and it contained the spiritual powers of the 7 greatest sages of Hinduism. 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. Therefore the Pandavas were forced to do cheating to kill Drona.-