Natasha Romanoff was a young Soviet citizen when her government chose to train her in espionage. During her time in the infamous Red Room Academy, she became the master spy known as the Black Widow.Marvel Source
Early in her career, the Black Widow clashed with Iron Man, but her loyalty to her country waned and eventually she switched sides and joined S.H.I.E.L.D. She later joined the Avengers, and has served with distinction on that team many times.
Blackwidow doesn't self regenerate so Occulus and the relic of Lemuria are their to compensate.
This is a spirit heavy build which is why Pyms Shrinking is needed. If you don't have it, you might want to swap the Lemuria for Gibborim. One point was put in the range basic for those times when you have nothing in the tank.Since criting alot, with pyms when you dodge and basic when needed should keep the spirit up.
For medallion selection, i chose Hyrda. However, alternative medallion choices can be Pyro or lvl 60 black cat or bulleye now. It depends on how good your spirit management is.
As for Legendaries i would recommend Gungnir as you would get most bang for your buck and allows flexibility if you change your build later. Alternative if your happy with specialising in energy dmg get the Cosmic Control Rod.
The build and artifacts are what i choose to help balance out the weaknesses of black widow, namely she is too squishy right now to get too close and melee. And that spirit management can be a problem. They are just suggestions and not a definite answer. I just thought they worked well together. Feel free to put whatever artifacts you like. This does limit black widow to a four move character but i've found this useful since i wanted to go ranged and keep crap off me. I may drop adv Super soldier for adv meta but haven't made up my mind.