You can do this at almost any point during the story, but if you do it early on, it's a lot easier to waste your health as you won't have any upgrades and require far less hits to lose your health. Once you encounter the area with the first boss like enemy, allow it to waste your health until it's down to a small red flashing bar. Once you've got to this point, walk up to the enemy, then hold "x" as the enemy tries to attack you. If you time the attack right you will parry and this will unlock. You can do this on any enemy in the game, but the bigger enemies are far easier to time.