You asked us to explain in the comments so here it is. His power is anti magic like you said, he didnt use strength to repel that attack (strength wouldnt have worked), the flat side of the sword repels magic the bladed side cuts through it (as they said). So when he fought that guy in the wizard exams it wasnt that his shield was weak or Asta was just that strong. It was just that magic is useless against Asta (or against his swords to be exact)