A marksman was a title for an individual who specialized in firing weapons with extreme precision.
Commander Spock regarded Captain Koloth as a proficient marksman. (TAS: "More Tribbles, More Troubles")
Jedda, a scientist on Regula I was killed by Captain Clark Terrell, who was under the influence of a Ceti eel put into his ear by Khan Noonien Singh. After Terrell committed suicide and his first officer, Commander Pavel Chekov, who was also under the influence of a Ceti eel, was incapacitated, Admiral James T. Kirk grabbed Terrell's communicator and yelled at Khan "You've managed to kill everyone else, but like a poor marksman, you keep missing the target!" (Star Trek II: The Wrath of Khan)