Corvus is the character that teaches you combat skills (such as blocking and counters) at certain points of the game. Keep an eye on your quest list. He will occasionally pop up as an active quest at various points throughout the game. Just visit him a total of four times (he is located in Lucy and Everett's backyard in the future in Old Auburn Pines) and master his skill each time. After the fourth time, the achievement unlocks.
Good luck!