The B.B. is a very stealthy, very hit-and-run type of Grimm who's strike from the shadows and generally never give you a chance to actually harm it. In a strait up fight it gets by thanks to it's unique amorphous nature, but if you can get a solid hit onto it it's rather weak. Hence why it's a sneaky attacker who doesn't get into strait up fights when possible. Physical attacks are largely useless, Elemental or magical however will prove to be the best bet to take it out. Things like Flaming or Electrically charged weapons will allow Physical Attacks to do harm to it, if only thanks to the elemental properties. Remember, B.B. is a boss level Grimm. He ain't suppose to be easy to beat.