She did almost all of the fight sequences on her own, but Heidi Moneymaker was her fight choreographer/trainer/stunt double (but only for extreme stunts).

Scarlett Johansson insisted not to use stunts for her role - Black Widow, a highly trained spay in The Avengers. However, it is not easy to perform all the fighting scenes as Scarlett had to go through lots tough training and many injuries.

Once she commented her role by saying "For The Avengers, I’ve spent so many months training with our stunt team, and fighting all the other actors, it’s crazy. I do nothing but fight—all the time."