Well, since that Oni is the only one alive, and all the vampires are treating him like the boss, it's safe to assume that he is the boss. The fight will end after he dies; the vampires have already said they won't fight and Gantz only targeted the Oni aliens. The vampires are next up on Gantz's agenda, though, we have been lead to believe.

As to the Horsemen, I assumed that there was no boss; they were all equal in strength. Tae counted as the boss. Either Gantz killed her because of her camera, or Gantz killed her because he's a bastard. If it's just because he was a bastard, then she was the boss. Otherwise, it was one of those missions like the one Kurono did alone where all the aliens are of equal strength.