Mardu Midrange: This deck focuses on buying time while setting up a combo of Rabblemaster and Butcher to continue feeding Butcher and eventually taking the game.
Brimaz: Creates Cats to sacrifice to Butcher (they're his favorite). After about 2-3 turns of this, if you haven't won, you're doing it wrong.
Rabblemaster: Same purpose as Brimaz, but you'd prefer to have him if you're still early on in the game, this is so that if you have a turn 3 rabble, you can sac a token to a turn 4 Butcher for haste, and then turn 5 for lifelink.
Stormbreath: Honestly, you're gonna run out of steam sooner or later, so this is added in as a finisher just in case. I might add in Mardu Ascendancy if I can find something to take out so that I can have something to fuel and help this snowball. (also, it helps Rabblemaster and Butcher in a pinch)
Despise: Mana heavy hands hate getting hit by this, disc a Xenagos from G(r) devotion, or maybe even some small critters in the early game, but the best move I've made so far is avoiding getting hit by Hornet Queen and her minions before they ever hit the board.
Elspeth: If I'm having issues feeding Butcher, her +1 is excellent (also good for snowballs). Her final is also a good way to seal the game; the only problem I've seen so far is that if I'm using her final, I've already all but won the game.
Sorin: All of his abilities help in a pinch. Unlucky hand, but still able to Butcher on turn 4? Turn 5 you just drop sorin, make a token, and begin both buying time AND pushing for game with Butcher.
Whip of Erebos: If I'm in the later game and my board is having issues or I need to feed Butcher, i can just Whip a creature back from my grave and seal the game with Butcher. (If you have Butcher and you whip Rabble or Brimaz, you can attack with the board, sac the token, gain 8-9 life in the process)
Lightning Strike: Helps deal with most issues in the format like rabblemaster or queen hornet even though it's not quite enough for Courser, but that's why we have downfall.
