Arjuna had trained himself under Drona as a great archer. Drona had given him training on each and every weapon. Not only this, sensing a sure war between Pandavas and Kauravas, Krishna sent Arjuna to Heaven to acquire all divyastras, so that he will be able to counter Bhisma, Drona and Karna in the war.
But, why was Arjuna not able to counter the snake weapon shot by Karna; which meant a sure and certain death of Arjuna, had not Krishna{as avatar of God Vishnu} put his weight to push the chariot down within earth?