Someone who attacks or assails another violently, or criminally.
(figuratively, by extension) A hostile critic or opponent.
1782, Frances Burney, Cecilia, London: T. Payne and Son and T. Cadell, Volume 5, Book 9, Chapter 3, p. 41,
[…] the assailants of the quill have their honour as much at heart as the assailants of the sword.