As a comment posted to another participant before my comment here to you pointed out, attacking a witness's statements on the basis that the witness may be biased, or may not have a basis of knowledge for what the witness said, is all standard procedure in a court case. The rules of trials allow the finder of fact to be informed of why not to believe everything that is said during the trial. It's up to each side in the litigation to follow the rules of evidence (a THICK book, interpreted by thousands of court cases) to suggest why some evidence presented at trial is better than other evidence and more useful for truth-seeking.