Seamus is one of those masters who doesn't seem to have a weakness. Rasputina is slow and needs to avoid melee at all costs. Lilith is slightly more fragile than other Masters (if you can hit her) and has nothing in the way of ranged damage. Seamus on the other hand has average to high stats across the board: an average defense, a high willpower, average speed, 12 wounds... And that's before we get to examine his abilities.
First off, Seamus is Terrifying (All) 12. Let that sink in. Everyone finds Seamus terrifying, even Rail Golems, Mature Nephilim, everything that doesn't explicitly state an immunity to Horror Duels is a target. Anyone wanting to target Seamus needs to pass a horror duel or fail their activation. This is going to cause your opponent to have to cheat a lot of cards from his hand to do anything to Seamus, even to get close. Remember, the beauty of the horror duel isn't to stop the opponent, that's an added bonus. The main advantage is making your opponent flip cards, and they'll likely have to cheat a few times on bad flips. You're causing your opponent to discard some of their best cards to keep moving.
Assuming they get past Terrifying, an assailant then faces Impossible To Wound, significantly reducing their chance of high damage output as the damage flips cannot be cheated. On the frustratingly low chance of a decent damage flip, Seamus is then also Hard To Kill, which is going to keep him around just that little bit longer.
Oh, and just to keep it Seamus-style hilarious? Feast of Fear means that every time an enemy within 6" fails a Wp duel, he heals those hard-fought wounds right back. Oh yes, Seamus is one heck of a tough cookie to crack, and we've not even looked at his actions and upgrades yet - we're still on the front of the card...
But what about the back of the card? The hilarity continues in one of the most amazing ranged attacks in the entirety of Malifaux, his .50 Flintlock. At Sh 6 and a range of 10", it's surprisingly accurate, and then it's damage output is 4 at lowest, and racks up through 6 to 8! If you manage to get a Red Joker in on the damage, that's an absolutely hideous TWELVE damage. Yes, I'm spelling the number, it's that significant. 12 damage is enough to slay most Masters outright, and 6 or 8 will do in most minions and enforcers. If you really want something dead, take aim: use Focus. The .50 Flintlock can only be used once per Turn, so you really need to make it count. Do not allow anything less than a Black Joker ruin your fun.
Backhand continues the fun as a melee action. It's much lower damage than the .50 Flintlock (but then, isn't everything?) but is repeatable on trigger, and the humerous Bored Now trigger allows you to push an enemy away, which is great if somehow they've managed to pin Seamus down when you'd rather he was off having fun elsewhere.
Let's examine his Tactical Actions next.
Where would a Resurrectionist Master be without the ability to raise the dead? In the wrong Faction, that's where! Arise My Sweet is a zero action that summons a Belle. Yes, it's tough to get off, requiring 9 and Crows, but a good cheat and/or a burnt Soulstone is all it takes to start overwhelming your opponent with shambling zombie hookers. If you're opponent is being so boring as to hug cover, why not Lure him out with a Belle and show him just how much fun a .50 Flintlock can be!
Finally, we have Back Alley. For a cast of 7 (or 7 Tomes for the trigger) Seamus can jump somewhere within 10" (16" on trigger). It's no use for running away, as it requires Seamus to be out of LoS from any enemy (though you can always Lure him or the enemy into such a situation) but it is great for jumping to the other side of blocking scenery where your opponent thinks they are safe.
So far you can see that Seamus is notoriously hard to dispose of, and has a habit of turning up where you least expect him to pop a .50 Flintlock in your back. That's just from his main card. What about his upgrades though?
First up is Sinister Reputation, which means enemies within 6" have a lower willpower. What's that? Doesn't Feast of Fear also have a range of 6" and doesn't it rely on willpower checks? Why, yes! Yes it does! And isn't Seamus Terrifying (All) 12, and don't horror duels count as willpower duels? Can you see where I'm going with this? On top of that, it also gives him Live For Pain, a 10" range casting shot that allows him to heal equal to the damage he deals, helping keep him constantly on top form. It's not .50 Flintlock on damage, but it heals, and you can use both in the same activation.
If you reckon Seamus might get up close and personal, his Bag O' Tools upgrade will keep you cackling. It has a 2" range, has a more than respectable Ml of 7, and a decent enough damage line, the fun comes in the triggers, one for every suit. Making an enemy model Insignificant can be a game changer. Dropping a scheme marker mid-combat is always useful (and this ignores the usual scheme marker placement rules so can be within 4" of other scheme markers). If you want the Sweet Murder trigger, you're likely going to need to cheat in a Crows and burn a Soulstone for another one, but forcing your opponent to discard two cards or Soulstones or die? No matter what they do, they lose.
Red Chapel Killer is a very different kettle of fish. It boosts his Back Alley action to allow him to jump to a scheme marker rather than just terrain, and has the Distraction zero AP ability to discard a corpse marker to ignore the "must be out of LoS" restriction. Great for those little jumps.
Then there's Mad Haberdasher. Just when you thought Seamus was hard enough to kill, he has an upgrade that he can discard to ignore any damage your opponent has finally managed to get through, and drop a 30mm hat marker. Next turn, Seamus can walk over, pick it up and continue, or leave it there for someone else.
A word of caution though. Any model can pick up the hat marker, including enemy models. Beware.
Not Too Banged Up increases the potency of your Belle's Lure ability. Remember that you can Lure Seamus too, anyone is a target for it. This makes Seamus' crew one of the outright most tricksy when it comes to movement effects. Lure models out of hiding, Lure them off cliffs. Heck, you can even Lure your own models further into the playing field.
Then let's look at the generic upgrades. Unnerving Aura punishes your opponent for staying close by. Honestly, it's tempting, but there are better options for those slots. Seamus isn't going to be tying people up in combat when he can be running around murdering for the sheer joy of it.
Decaying Aura suffers similar issues. Seamus isn't going to be close enough most of the time for this to pay off.
Corpse Bloat does have its uses. The Unforgiving Stench action causes a Wp duel, target a nearby corpse marker and Seamus might get some heal for some damage through Feast on Fear. The zero action allows him to place a corpse marker by taking damage that he can then heal, so if you intend to be summoning lots of Belles, this upgrade my find some play.
There's a running trend here, I still don't find the Stone upgrade worth it. Asylum Stone does have a semi-decent zero action, but the Df/Wp boost just still doesn't strike me as making this card worth the upgrade slot.
Maniacal Laugh allows you to summon some Mindless Zombies, but I don't see why these would be much use beyond chaff, and Transfusion is pointless when Seamus doesn't utilise Poison.
So that's Seamus, but what about his crew? Certainly, as usual, the Shadows of Redchapel box is your best starting point. The Copycat Killer is the only real totem I'd bother with for Seamus as he twins so wonderfully - the Graveyard Spirit has limited use to make Seamus even more resilient, but I don't see much point in the Malifaux child or Student of Conflict here.
Seamus loves his Belles, and his Belles love Madame Sybelle for how she boosts them. I've considered Mortimer for his Fresh Meat action before to draw your Belles closer to targets, though.
Nurses have a limited use in their Hallucinogens, Uppers and Downers triggers.
I really like taking Killjoy with Seamus. He creates a decent melee beat stick and with the .50 Flintlock, it's a relatively simple affair to unbury him very early on right in your opponents half of the table. His Hooked Chains allow you to drag more enemies out of hiding for Seamus' .50 Flintlock, and Killjoy can heal himself easily enough. At 13ss to hire, he's expensive, but worth it.
But how does one beat such a tough monster of a man? Simply put, you don't. Target his Belles and other movement tricksters then keep to cover, but in LoS to stop him using Back Alley, preferably also keeping out of range of his .50 Flintlock. Be aware that, like Lilith, he has a lot of movement trickery in his crew. Expect to be pulled around a lot with Lure and the like, but hammer away at his crew. Unlike other Resurrectionists, his strength does not lie in summoning more undead, Seamus needs to use what he has carefully. Excise away his resources and you can win.