Honestly, I don't think that Natasha stands a chance against Xena. I'd give the Warrior Princess substantively better than even odds against Cap.
Guns might even the odds a bit, but Xena shouldn't get caught off guard due to the common knowledge clause of the rules, and Xena is probably sufficiently superhuman to aimdodge until she close enough to guarantee a hit with a thrown weapon or even get into melee range and just punt Black Widow into next week.