After the intro when you ride a Griffin, you need to do so a second time. All across the land, Griffins tend to fly over and often come down to fight. To earn this achievement, you need to be holding onto the Griffin (RT on Xbox controller) as it tries to flee in its low HP state. Hang onto it as it flies away and after a few seconds, the achievement will pop. Not sure if there's a specific time requirement you must be airborne on the Griffin, but it didn't feel overly long for me.