How to play- remove first, ask questions later (usually). Lots of 4 ofs make this a really consistent and reliable deck.
=========Creature Types
Butcher- Hits for 5 in the air as a 4 drop- all that you need. 4 turn clock alone. With a little help from tokens, haste and lifelink. Vigilance not too relevant usually.
Rabble Master- Comes out with a Goblin buddy, usually instant value. Early true aggro that gets out of hand like crazy.
Soulfire- Early game, heal off the damage you do to yourself. The extra health gain from the spells really makes a difference in certain matchups. Played over seeker, only because late game drawing seeker is always a luckluster play. Also this card fits in the- T2 soulfire, T3, Rabble, T4 Butcher curve better without needing prowess for lifegain. If you draw this guy late- use crackling doom to remove their heaviest hitter with buyback, everyturn. Sideboard? Glare, peak eruption, or downfall- everyturn. Relevant early and late game- mid game- have other things.
=========Spell Types
Chained to the Rocks- Excellent removal, usually no problem finding a rock to tie things down. 5 fetches, 5 basics.
Lightning Strike/Magma Jet- With Soulfire out- your bolts become lightning helix early, and have buyback late. Kinda mixed feelings with magma jet as the Scry 2 really helps smooth out the deck. If you want to go 2/2 strike/jet it makes sense- depends on your meta. Jet hits things like seeker, rabble, death dealer, etc. very common early game, but strike hits Mantis rider, lion, lets you kill a T3 Ashiok with a swing too. (which is a pain for this deck.) Hordechief, untributed fanatic of mogis, etc. Strike also lets you finish off all the higher toughness mobs too with either a goblin token or Soulfire himself. Basically- reach is good.
Crackling Doom- Anti-stormbreath, anti-heroic decks, anti-indestrutible hexproof things (i.e. fleecemang). This card basically says- idc get rid of your threat. Great removal and damage, and with soulfire healing/removal. A very solid to go Mardu over Boros.
Mardu Charm- An unsung hero in Mardu. An instant speed Duress, invaluable against control decks. EoT- discard a counter spell from you hand, into your turn play a threat. The instant speed tokens cantrip better than Hordeling outburst. You do lose a token playing this over Hordeling outburst- but you get a card that is actually easier to cast in Mardu, instant speed, options for removal, cant bile blight your goblin tokens AND your Rabble tokens, a removal option if you know your tokens are going to die, an instant speed play into lifelink, haste Butcher, combat trick soldier that can kill an enemy rabble/no prowess Seeker/ bye goblin tokens/bye any 2 toughness attack/ i didnt see that blocker coming- oh no my lethal math is now off. Lots of utility and synergy in the deck and I keep coming back to it- satisfied its in the deck.
Thoughtseize- Lots of Midrange and control decks running around, this card puts you ahead of that plan with a turn 1-2 beast that sets up the game in your favor, in a strong way. The 2 life loss isn't too big a deal as you gain 5 safely swinging with a soulfire into a lightning strike turn 3.
Tormenting Voice- IDK about you- but I draw a lot of land I don't need. Tempting to put in a Crater Claws. Throw em away and draw or fix your early game with this card. Facing control and don't need chain? Toss em away.
=========Walkers
Sarkhan- Another card that has surprised in testing, If stormbreath could Mizzium Mortars. This card cant be burned down or downfalled the turn it comes into play, and will almost always get off at least 4 damage to either your opponent, or a creature. Another plus to this card- is if you decide to wrath or Anger- He isn't phases. This beast just pluses and continues to attack. Also- if you remove enough you can get super value out of removing their Brimaz or Courser, than on your next turn start flying in for the kill. Really awesome, the Emblem is kinda relevant, you'd probably rather just kill the opponent. But if you want to toy with them like a dbag, than by all means emblem.
=========Land Base
9 untapped(10 with evolving)- wouldn't do more, would do less but Outpost is crucial and if you're not playing magma jet you need the card fixing. 5 Mountains and 5 potential Mountain fetches lets you safely play your playset of chained to the rocks.
24 lands- Most of the deck is played at 3 mana. There are 8 cards above 3. You can get buy on that. Later the 4 and 5 open up the win cons, with a single 6 mana elspeth. Maybe 25 lands with an extra scry, meh. I always seem to get enough lands.
1 Evolving Wilds- No arid mesa or marsh flats = no plains search in Mardu colors. next best thing- evolving wilds. Need a mountain for chain? Another Mountain fetch. Need that swamp or plains still? Go for it. It's also fuel for Murderous cut if you want to Sideboard that against your creature heavy meta.
1 Swamp- Don't need to much more than that. Pretty much never need another swamp aside form Downfall, and nothing in black will need to be played T1 THAT consistently.
2 Pains- Dont want too much, but dont want too much untapped lands.
5 fetches - filter your deck! Basically after 4 lands you dont need anymore (but the deck can still make use of em.)
SIDEBOARD
This varies a lot depending on your meta.
Cards to consider are---
Anger of the Gods- early board wipe. Especially good vs certain green decks, phoenixes, Boros aggro, and token decks. (Also gain massive life with a soulfire out- heheh.)
Crux of Fate(or end hostilities)- When you're on the draw especially, you can get behind other strong decks pretty easily. Wraths are what make the difference. Bring em in, shut em down. Sometimes the difference between winning and losing. Also- 'single' target their stormbreath too if need be.
Glare of Herest- This card hits a lot of high tier decks. Chained tot eh rocks, fleece mane, SEIGE RHINO, Anafenza, brimaz, seeker of the way, monastery mentor, and others- and EXILES doesn't just grave em. Gr8 h8.
Outpost seige- Your golden ticket vs control. Sometimes playing this in Khans means flat out beating control in card advantage. Dragons is pretty much used to do a last little bit of damage if REALLY needed.
Peak Eruption- If Boros aggro is rly popular, a couple of these bad boys not only blow up the rock chaining your few creatures in the deck down, but it does a crackling doom, put-you-ahead-oh-and-you-take-some-damage-for-funzies, but you can also gain life from soulfire (and this done on T3)
Deflecting Palm- Playing against big crater? Need to race your opponent? This card is good in matches you find yourself needing to race. Sometimes stopping a lifelink seeker and preventing the prowess bonus for a turn can really make a difference. Easily underestimated card. Always a good trump card no one sees coming. Oh is that a 10/2 rabble master? Take 10 instead. Oh is that a 10/10 Hero of Iroas? Take 10 instead.
Murderous Cut- Easily casted for cheap. You can bounce this card for 5 mana total with Soulfire. Not bad.
Thoughtseize/Despise- Against a deck running all creatures for aggro purposes or devotion, sometimes not hurting yourself with thoughtseize and taking that Ugin, or early card bomb can give you just enough time, while still seeing their plans.
Hordeling Outburst/Raise the Alarm- This deck is fully capable of just having a sideboard alt win con. Swing wide with lots of tokens! (worth considering with Brutal Hordechief, too) Sometimes it's not about having the right hate- but getting around what hate the bring to you. Depends on the opponents knowledge a lot. Probably best saved for FNM.