The rule that slander cannot be justified, even for a constructive purpose, would seem to be without exception. The Chofetz Chaim, however, speculates that if one cannot bring about a given constructive result without exaggerating, then it may be acceptable. The case he cites is where one is attempting to dissolve a friendship between two children, one of whom is a bad influence upon the other. If the victim will not be influenced to end the relationship unless the evils of his friend are exaggerated, this may be permissible.
The guidelines for such an exemption are very sensitive and one cannot rely upon it without prior consultation with a competent halachic authority.