I'm not sure what you all are doing to make burning eight cards a turn weak. That can completely ruin many decks.
Largely, a combination of two things:
1) The Harbinger is merrily tossing out cards from its prey. It potentially uses Trochomancy to oust its prey. The prey is ousted. New prey, and you have to start milling them all over again. Most games don't have
that many turns for each deck, overall. An interesting feature of milling is that until the deck is empty, it actually doesn't directly affect the target in terms of what they draw - unless you have library foresight or manipulation abilities (most decks don't), losing a few cards off the top of your library is the same as your deck just having been shuffled differently.
2) By playing Slaughterhouse, the Harbinger is not playing a whole truck of other really, really strong master cards in the quantities it could be. This will often give a predator an opportunity to strike because - in particular - you're missing out on pool gain, from Villein, Blood Doll, Vessel, Giant's Blood etc.
He uses the Slaughterhouse to power Trochomancy, which is +bleed by burning ash heap cards. All bleeds directed at him are redirected with Telepathic Misdirection or Telepathic Counter. Any attempt to enter combat with him is met with combat ends or fortitude. It's truly nasty.
So, there are a whole bunch of things you can be doing.
First, a very simple one - pack some Washes, or another anti-master tech of your choice. It won't neutralize him, but it can slow him down. Wash, in particular, is very little cost to you, and highly versatile. Your players should be playing strong master cards, so even when you don't see the Slaughterhouse deck, you can be reversing Villein, Dreams of the Sphinx, Pentex Subversion, Week of Nightmares, or whatever else. If several decks on the table see it as a problem, it might well lose several of its Slaughterhouses - and if the deck is so overwhelmingly strong that it destroys everything unstoppably, several decks on the table
should see it as a problem.
Along similar lines, it's a location. There are a bunch of ways of interacting with locations, and the deck as described isn't also a superlative intercept wall, so some of those actions can get through. Burn it with Arson or Conceal, steal it with Disputed Territory or Dominate Kine.
The deck as described has a bunch of natural holes. Stealth-vote isn't susceptible to Telepathic Misdirection or Telepathic Counter or Combat Ends or Fortitude. Obfuscate Ventrue can do it very well indeed - throw in some Parity Shift, KRC, maybe a couple of Disputed Territory or Banishment. Mix in some Mind Rape for fun and profit. Stealth-anything has the potential to do well.
The deck as described is relatively toolbox-y, in that it has a bunch of Telepathic Misdirection, a bunch of Telepathic Counter, some Fortitude combat defence, some Combat Ends, some Trochomancy, and Slaughterhouses. The thing with toolbox decks is that they have a whole lot of breadth but often not a lot of depth. You can try to make it run out of certain types of cards by focusing your tactics.
For example, it only has so many Telepathic Misdirections and Counters. Bleed little and often - and bounce bleeders there yourself, if you can. Playing weenie decks can help there. Bleed a lot of times using very few resources, and do it again next turn. Weenie Fortitude can be a lot of fun here, as one option.
You mention Combat Ends. Any significant combat deck shouldn't care too much about Combat Ends - Immortal Grapple, Psyche!, Telepathic Tracking, Sanguine Entrapment. You should be able to prevent or whittle away the combat ends easily in any fairly serious combat deck.
Fortitude combat defence is slightly trickier - but again, it doesn't have infinite of it if it's packing all the things you say it's packing. Look at the Fortitude combat defence it chooses and try to play in a way that upsets it. If it's playing damage prevention that prevents a set amount of damage in one go - say, Skin of Rock or Resilience, or that sort of card - consider going for additional strikes. Celeritous gun decks can plough through S:CE with Psyche! and gain an additional strike when it doesn't face an S:CE. Relative weenies with Concealed Magnums is playable. If it's using more cards like Skin of Steel, or cards with blood costs, you can potentially go to multiple rounds - Psyche! lets you press, Telepathic Tracking lets you press, Immortal Grapple (superior) lets you press. Or if it's Skin of Steel type cards preventing damage from strikes, look at doing non-strike damage - going back to weenies, weenie Animalism can be brutal with Carrion Crows and Aid from Bats. Sure, you waste a few bleed actions to Telepathic Counter or S:CE, but it can't do everything all the time or else it will never draw Slaughterhouses, Trochomancies and cards it needs to get its bleeds through - and with a well played weenie deck, you should out-minion it.
Most Harbingers decks are slower to start, as their smallest vampires are 5 caps, and any given Harbingers deck could easily find itself with an uncontrolled region that is quite a bit bigger than that. Capitalize on the speed - weenie Legacy of Pander, weenie Dominate, weenie Dementation, or any deck that can get off to a flying start. Such decks can potentially go very aggressive and have done a
lot of pool damage before the Harbinger even has its first vampire out. Sure, the deck might not be able to sustain that - but that's okay, you've got the unstoppable Harbinger deck off the table, gained 6 pool, and can maybe catch your breath next turn.
The Slaughterhouse
NC Russia
