While not a full build, Douglas Cole of Gaming Ballistic did a workup of Black Widow's martial arts skills in action from a scene of
Iron Man 2. This should help you figure out how she takes names and kicks butt (she needs a clipboard because there is so much butt to kick!)
You get an idea of her skills. Given that Black Widow is not superstrong, higher Striking Strength and high Karate skill should be how she does her damage.
https://gamingballistic.com/2013/10/...widow-in-iron/