The B Rank Missions are only assigned to to experienced ninja. With these missions you are able to unlock villians and subordinates of the most dangerous ninjas.
Win 5 battles with Uzumaki Naruto versus Haku or Inuzuka Kiba. (5/5) Win 4 battles in a row with Uzumaki Naruto. (4/4) Win 5 battles with Haruno Sakura versus Dosu Kinuta or Yamanaka Ino. (5/5) Win 4 battles in a row with Haruno Sakura. (4/4) Win 5 battles with Uchiha Sasuke versus Gaara of the Desert or Uzumaki Naruto. (5/5) Win 4 battles in a row with Uchiha Sasuke. (4/4)