What claim did Abigail make in order to destroy Mary Warren's testimony?
I assume that you are asking about the part in Act III when John Proctor has brought Mary Warren back to the court to say that she and the girls have been making it up when they have accused other people of witchcraft. They have been pretending to see spirits, etc.
Ironically, what Abby does is to see a spirit. She says that Mary is sending her spirit out in the shape of a huge bird. She pleads with Mary to stop. All the girls back her up and Mary's credibility is gone.