Instructions
Play through the game until you come across a centaur. These are normal enemies and not a boss.
Attack the centaur as normal, pumping the "Square" button until the other enemies around you scatter away. This means the centaur is about to attempt spearing you.
Move your left analog stick in the direction that appears onscreen when prompted. For example, the game will either tell you to move the stick in a counterclockwise or clockwise motion, and performing the action successfully counters the spear.