Arguably good for the soul, but undoubtedly terrible legal advice:
Tell it like it is.
In the guide Cupids Messenger of 1629, the anonymous author told his readers how to write to an unfaithful partner. There was really no point being polite. Far better to be bilious and vengeful. "Leprosie compared to thee is all health... neither thy bodie nor thy soule are free from the disease of shame and disgrace of the world." Try this ancient advice first, and only then revert to the more modern solution of a letter from a solicitor.