Krishna warns Arjun before the decisive fight that Karna's bowstring cant be cut. Arjun decided to aim for Karna's body straightaway.

Krishna divined Arjun's intent and warned him that Karna was none other than his foe Sahasrakavacha from his previous birth as Nara. https://twitter.com/AndColorPockeT/status/1253986001464123392

Krishns asks Arjun to go in yogic trance and recall how he as Sage Nara had shattered armours of Sahasrakavach (Karna in his previous birth) by hitting the armour with arrows real hard. Arjun recalls his previous birth and how he as Sage Nara had shattered Sahasrakavach's armour

The decisive fight begins. Both destroy each others' supporting warriors and forces. Karna cuts Gandiva many times. Any warrior losing bowstring to Karna means he is history. But not Arjun. Arjun had kept multioke strings on his bow. Every time Karna cut Gandiv bowstring, ++

++Arjun replaced it in a jiffy and showered arrows on Karna. Arjun was very light-handed and could operate the bow very fast (sheegrayodhi ..other fighters of this type ..Bheeshma and Abhimanyu). Karna thought tgat Vijay bow cant be cut but Gandiv bowstring can be cut.

So he had calculated that if he cut Arjun's bowstring, Arjun's head can be made to roll with the next arrow. But that was not to be. Arjun had many bowstrings!!!!
Then he is unable to prevail on Arjun. Arjun was beginning to prevail. Immediately he invokes his ++

++"sharam ghora sarpamukham jwalantam" "terrible flaming arrow with head of a snake". No his serpent dart was not Nagastra/Nagoash like that of Indrajit. This was a Snake Aswasena with enemity towrds Arjun who transforms himself i to an arrow and sits in quiver of Karna ++