Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
impugnātor (inp-), ōris, m. [impugno], one who attacks, assails (late Lat.; in Liv. 27, 15, 8, the true read. is oppugnatores), Cassiod. Var. i. q. Ambros. in Psa. 118, Serm. 13, 6.