I can't vouch for Griever or Ultimecia's immunities either way.

I doubt it's BGH251F2 since Iron Clad was already and incorrectly guessed.

That leaves Blue Dragon, Cocatrice, Elvoret, Forbidden, Gerogero, GIM47N, Hexadragon, Iguion, Omega Weapon, Tri-Face, and X-ATM092.

I'll guess X-ATM092, since it's nicknamed the Black Widow.