I have yet to play a game with both butcher and hunting party, but here are my thoughts.
While HPX is normally paired with a 3-4 cost card (baron, monument, swindler etc), it also is useful with certain 5 costs. Mountebank is one, Counting House has the potential of being another, though it has no other upsides besides near-guaranteeing 8, stash and royal seal work in a pinch, and there are a few new compelling Dark Ages cards. One is Rogue, which has the potential of trashing the opponents HP's in a mirror, and the other is count, which can store excess HP's on top of the deck, and if you happen to hit 8 without needing it's +3 then you can duchy province for fairly cheap.
Of all the 5's there are a few that really stand out. Mountebank(duh), Butcher, Count, and Rogue. Whether these overcome the loss of tempo it takes to gain them is debatable, but I think they are all certainly strong options to consider.