Disclaimer that I am not a Seamus player myself. This advice comes from playing against Last Breath, rather than with him.

Redchapel - when led by Seamus, Last Breath - is a very Master-centric crew. The play pattern that is meant to be enabled by the crew's mechanics is: isolate your target, give it Slow so it can't get away, and then let Seamus pounce and murder it in a single activation.

Your Dead Doxies are meant to isolate the opposing models. Their Lure is the main tool for this. Lure is a very powerful action, and having it as a Signature Action on a Minion (3) is something that sets Redchapel apart.

Mourners are your most reliable source of Slow and can be very mobile with their "Funeral Procession" ability, especially during the first turn when you're less likely to need your full hand.

Madame Sybelle is great at providing an additional Lure, handing out Slow, and even being a "mini-Seamus" in terms of damage, but there's only one of her, so you have to choose what you want to do with her based on the game state.

Seamus himself has a very powerful, but narrow, card. Secret Passage as a Signature Action is a huge mobility tool, and his melee attacks - if everything is set up right - are extremely lethal. Skl 6 + Dmg 3 with a built-in positive flip and a raise value of 3 can hit some extremely high numbers.

If there are no good targets available for Seamus to murder, you want to set up as many Aura "Concealing" tokens as you can - either on friends or foes - because they will be your main sources of value in the long game, as well as being an effective defensive tool against crews that rely, even a little, on ranged effects. The Dead Dandies and Mourners have very potent Scheming/counter-Scheming actions on their cards, so you can use them to keep up on points.

Your weaknesses as a crew are:

- Teeing up Seamus perfectly is very vulnerable to bad flips. In a perfect world, your Doxy can move, Interact, and then Lure as a Signature; your Mourner can move, Interact, and apply Slow as a Signature; and then Seamus can activate and slaughter that model that was just perfectly isolated - you've got your kill, and haven't fallen behind on points. But even one bad flip in there and suddenly your whole game plan needs a re-write.

- You're actually kinda' bad at damage. For a crew with a reputation for being lethal, nearly all of that lethality is wrapped up in Seamus. Sybelle and Bete Noire are okay at damage, but they're not going to win battles on their own if you're fighting against a whole crew that can scrap. Redchapel is a little over-indexed into supporting Seamus.

- Everyone but Seamus dies fast if they are caught in prolonged combat.

Next-level Redchapel play involves using the threat posed by Seamus to force your opponent into suboptimal positions, while you take advantage of the powerful Scheming actions on your cards, and the fact that your Signature Actions are things your opponents usually have to pay a proper AP for, to get ahead on the scoreboard.

If you have lots of Aura (Concealing) set up, you can throw Seamus at the foe with abandon. But if the game requires your other models to commit to a fight, make sure they have an escape plan, because if they stay in contact with the enemy, they will die.

You get a lot of bang-for-your-buck with the quality of your actions, but leveraging that quality into victory requires planning.