So, what's your favourite boss to team up against? Mainly focused from the point of view of you as a phantom, rather than the summoner.
For me, I'd say it's either O&S or Capra.
Against Ornstein and Smough, it's just a free for all brawl where the host is trying to survive, while you are trying to distract one of them. It just feels right.
And Capra? Welp, he starts the battle teaming up on you, so why shouldn't I return the freaking favour?