Story related and cannot be missed.
This unlocks after defeating Hercules at the end of the second episode, "Rhythm of Olympus". I used a combination of blocks, punches and ranged attacks to fill my meter and then I used my Heroic attack when it was available. Hercules will occasionally grab a column and try to smash you with it, so just move out of the way. Once the platform starts to rise, Herc will block and try to shoot you with lightning. Just use another Heroic attack to finish him off. If you do get knocked out, just run over and revive the fallen character with .