As both of these series’ main characters are from the same guy, it’s only fitting that they fight each other. Haiji can manipulate gravity to a powerful degree but Asta’s already fought someone like that before. Haiji will not be able to make Asta bend to his will. Asta’s anti magic is just too powerful and in addition he has several super forms at the ready. Haiji’s going to be on the defensive and will quickly be defeated. Asta wins.