Who trained Karna in archery and art of Warfare after Drona denied to teach him & latter cursed him?

Parashurama is the correct answer. After Dronacharya refused to teach Karna (due to his low birth), Parashurama, who hated Kshatriyas, taught him advanced archery and warfare. When Parashurama discovered Karna was a Kshatriya (not a Brahmin as claimed), he cursed Karna that he would forget his skills when needed most. Ekalavya was self-taught. Krupacharya was the Kuru teacher who initially rejected Karna. Balram was Krishna's brother, not Karna's teacher.