“檄文执笔者” can be translated as "the writer of a manifesto (or a proclamation/call-to-arms document)" in English.
Here, “檄文” is a traditional Chinese term referring to an official proclamation or call-to-arms document, often used in ancient times to rally support or declare war. While there isn't a direct one-word English equivalent, “manifesto” or “proclamation” can convey a similar sense, depending on the context. If emphasizing the militant or rallying aspect, “call-to-arms document” could also be appropriate.
For a more literal and context-specific translation, you might say "the author of the proclamation/manifesto" if referring to a specific document.