In mahabharatha Karna's Kavacha and Kundalas made him invincible in battle.
Kavacha armour protecting his chest.
In which way kundalas protect him which are in his ears ?
It's because Kavacha and Kundala of Karna contained Amrita in them as described in this chapter of Vana Parva of Mahabharata:
Thereupon Kunti said, 'If, O dispeller of darkness, I obtain a son from thee, may he be furnished with a coat of mail and ear-rings, and may he be mighty-armed and endued with great strength!' Hearing these words of hers, Surya answered, 'O gentle maiden, thy son shall be mighty-armed and decked with ear-rings and a celestial coat of mail. And both his ear-rings and coat of mail will be made of Amrita, and his coat will also be invulnerable.' Kunti then said, 'If the excellent mail and ear-rings of the son thou wilt beget on me, be, indeed, made of Amrita, then, O god, O worshipful deity, let thy purpose be fulfilled! May he be powerful, strong, energetic, and handsome, even like thee, and may he also be endued with virtue!' Surya then said, 'O princess, O excellent damsel, these ear-rings had been given to me by Aditi. O timid lady, I will bestow them, as also this excellent mail, on thy son!' Kunti then said, 'Very well, O worshipful one! If my son, O lord of light, become so, I will, as thou sayest, gratify thee!'
As it was made up of Amrita, it would make Karnas body immortal until Kavacha/Kundala were attached to his body due to effect of Amrita. In this way Kundala also protected him.
Kundalas (ear rings) are reflected the sun light or moon light to his enemy's eyes just like seeing sun .. so karnas enemy could not hit karna with there arrows... So this tiny help from kundalas karna is impossible to win from his enemy's