take out the sword, and stay near the beginning of the level, here will be enough enemies to make the challange. focus some enemies around you, and take a closer look to the fight. you have to hit the enemy to the left-behind of baumusu. if the enemy is close enough, baumusu literally will perform a backhand to the enemies face, using his (free) left hand. better hope the enemy isnt blocking this rare move. place 5 and its done, took me around n hour though...