The easyiest way to kill one is in Sword base (the second mission), start in bravo point (where you have to return to the base). It will be in the left of the entrance before the hunters, if it doesn't appears, it will be inside the base, in the room where the shotgun spawn in MM.
Is a bit different form the other B.O.Bs, it appears with random armor (from minor to field marshall) and its color is White (like an ultra)