"檄文执笔者" can be translated into English as "the writer of a manifesto or proclamation" or more specifically, depending on the context, "the author of a xiwen (a type of official or militant declaration in ancient China)".
If a more general or less culturally specific term is preferred, "the writer of a public declaration" or "the author of a formal announcement" could also be used, though these lack the historical and cultural specificity of "xiwen."
For a direct and culturally aware translation, "the writer of a xiwen" is most accurate, with a possible explanation in parentheses if the audience is unfamiliar with the term.