In every story mission there is a large gong to hit with a sensei. Doing so will start a Battle Gong challenge, where the challenging sensei must defeat several waves of enemies while confined to a small area. You must use the challenging character; switching causes the challenge to fail.
Also, this achievement only requires 3 wins in general, not unique wins, so a person could replay a level to play the same challenge over and over and still get the achievement.