If Arjuna is supported from behind by Vrikodara (Bhima), he is invincible in battle. But if Bhima is not there, he (Arjuna) is not a match for even the forth part of the son of Radha (Karna).
अजेयो ह्यर्जुनः संख्ये पृष्ठगोपे वृकोदरे। तमृते फाल्गुनो युद्धे राधेयस्य न पादभाक्॥