During the Olympus mission (episode 2) you will eventually get to an arena of sorts. Follow the onscreen prompts to kill the training dummies and you will be attacked by a large wave of dummies at the same time. Your partner will prompt you to use your heroic attack to destroy them. Simply press to activate the special move and it will take 1 circle of power to use. After using it once "Heroic Attack" will unlock. Immediately after this first large wave of dummies comes a second wave of dummies. Pull again to defeat them all and get "Heroic Hero!".