Catch up with a few mirror-universe Avengers near the Noir/Egypt/Empire border.
You'll be taking down squads of HYDRA warriors. The first group does have health levels, but nothing else too dangerous.
Ditto the second group, but the third has a few troops in silver armor, so be sure to swap in a 'fig that can deal with that.
Defeat this third and final group of baddies to unlock Hammer, and complete the mission.