Originally posted by: ...Diala...
It should be Arjun.. As mentioned post Abhimanyu Vadha.. As he says he did not teach him how to come out after piercing it, it should be understood that he was the one to teach the first half too..
"Sanjaya said, 'Thus conversing, those two, (viz., Krishna and Arjuna), entered their own camp. And they saw that the Pandavas, all cheerless, were sitting, plunged in great grief. Beholding his brothers and sons, the ape-bannered Arjuna became very cheerless. Not seeing the son of Subhadra there, Arjuna said, 'Pale is the colour I behold of the faces of you all. I do not, again, see Abhimanyu. Nor doth he come to congratulate me. I heard that Drona had today formed the circular array. None amongst you, save the boy Abhimanyu, could break that array. I, however, did not teach him how to come out of that array, after having pierced it. Did you cause the boy to enter that array?