Karna faced many limitations and unusual circumstances on his side and nobody to support him in his war against Arjuna.
1. On birth, was left by mother, in fear of society.
2. Raised as sootha putra and didnt had kshatriya credential as should have been.
3. Couldnt participate in Draupadi Swayamvara, otherwise Pandavas wouldnt have had Draupadi.
Before final war with Arjuna,
1. Arjuna had Lord Krishna as charioteer, but Karna had Shalya, who didnt like Karna.
2. Krishna requested Karna to join Pandavas as rightful elder of family and marry Draupadi as sixth husband only after declaration of war, otherwise, things might had been different.
3. During learning archery, was cursed by his Guru.Parashurama for being a kshatriya. He was stung by Indra as an insect.
4. Kunti asked him to not to use a weapon second time after Lord.Krashna informed Karna of his origin, only after declaration of war.
5. Karna would have killed Arjuna by sarpasthra on first use, if he followed Shalya's request to change the aim from neck to chest.
6. Arjuna had Lord Krishna, Kunti, Indra, Shiva, Hanuman and so on to support him in his war against Karna. Despite which Karna had chances to kill Arjuna.
7. Shalya left Karna and karna had to chariot himself at critical stage.
However, its Lord Krishna's blessings that helped Arjuna in his war with anybody and his reclamation as the greatest archer.