Hearthpwn

Deck Stats & Achievements
Stats Legend 1500 to Legend 200:
https://imgur.com/a/m1C3HvF

Around the top 200 Legend I was struggling and winrate dropped down to 50%, as the opponent quality increased, it's still fine to do 50% against good players tho, right?

I was also using the same deck for a 7-1 swiss record in Masters Qualifier to Seoul.
(*Just for fun and practice and no intention to travel there, I won 10 packs!)

Mulligan
Mulligan is not straigthforward, as you have a lot of early-game cards and combinations and you have to see how the cards you keep work together, in this deck you are likely keep several cards in your mulligan given you don't try to hard-mulligan for anything special.
I will try to provide you with general tips.

General Mulligan
Go by the general mulligan in every matchup unless specified otherwise.
Always keep regardless matchup: Acornbearer, Keeper Stalldaris, Sn1p-Sn4p,*Wispering Woods (with coin or acornbearer), if you keep Wispering Woods be sure to keep 1 card to buff the wisps.
Those cards will be specified in matchup guides only if you don't keep them.
Keep against most decks ("Tier 2" keep list): Crystalizer, Dreamway Guardians.

Druid: Assume it's a mirror.
Mulligan: Swipe, Crystalizer, Dreamway Guardians, if you already have solid minions keep Power of the Wild, if you already have 1-2 drops can keep Wrath (can protect Crystalizer from a 3:2 Panther, can handle Stalldaris, but not neccesary for much else).
Strategy: Boardcontrol is everything, as the player winning the board is the one to benefit the many wide buff cards in the deck, you pretty-much trade everything in this matchup.
If you struggle on board don't be shy using the buff cards for lower value than their potentional as if you lose board they only get weaker.
if you run out of resources coming into turn 8 be sure to recognize it, can take riskier gameplans to prevent losing to The Forest's Aid.

Hunter: Assume it's a mech, but it can be midrange too.
Mulligan: Nothing special requires a keep.
Strategy (Mech): Boardcontrol is the main objective, trade everything - even the 0-2 bombs, to prevent magnetic upgrades. when game gets late you can more reliably ignore bombs (Especially if opponent played Flarkmaster), especially if you can't deal with all of them anyway, take the safer risk.
Be alert for the opponent's burn plan, after taking the board be sure to play around Leeroy or suprising magnetic upgrades.
the secondary win condition for the Mech Hunter is combining Missile Luncher with Venomizer to checkmate you out of the game.
The tools to deal with this strategy, apart from killing him before he can get the cards or mana for it, are active deathrattles (Soul of the Forest, Sn1p-Sn4p),Druid of the Scythe, Wardruid Loti (And Savage Roar with it) and Wrath - if you won the board be sure to hold on to those cards/set-up the deathrattles.
First thing to prevent this is not letting a high-health mech to stick around, as both are magnetic, so apart from the swing being bigger the minion could get out of kill range.
Strategy (Midrange): Play around Unleash the hounds - better midsize minions over many small-size minions.
If you are ok trading with Unleash be sure you don't lose to Scavenging Hyena added to them.
Starting turn 6 expect the opponent to rush you with either a 5:5 Unleash the Beast or 4:6 Vicious Scalehide so you don't want to be too weak against that.

Mage: Assume it's Cyclone.
Mulligan: Wispering Woods even without having the Coin/Acornbearer.
Strategy: Apply pressure as early as possible, to either end the game or force them to compeletely deviate from their normal gameplan.
If a giant sticks it could be checkmate, so take risks accordingly.
They can hardly heal (just Zillax), so committing early Savage Roar can lead to Swipe + Hero Power lethal.
If you have several board development options, consider playing around Sea Giant - just not much as scaring yourself of this card can lead to very suboptimal plays you have no time for.
Preparing Stealth + Poison Loti can be a good play, but it is too expensive for mulligan keep.
Be alert of any minion, as they can suddenly go Khadgar + Conjurer's Calling (Yea, even on 2 drops, even on 3 drops) and that could provide them with a contestment opportunity.
They sometimes play Doomsayer so careful about using the Coin for a 2 drop.

Paladin: Assume it's Holy Wrath, can be Mech too.
Mulligan: Wispering Woods even without having the Coin/Acornbearer, Wardruid Loti.
Strategy (Holy Wrath): Apply solid Pressure while playing around their removals.
Loti as Taunt is amazing in this matchup, as they can't contest it with minions.
Strategy (Mech): Poison Loti is good to kill exactly the big mechs they try to get up.
just flood the board and snowball early, they have no Consecration so they can't really do anything expect creating big mech(s), which is a complete tactical disadvantage for them as the Druid strategy of having multiple minions and buffing generates greater value.

Priest: ???
Mulligan: Wrath, Druid of the Scythe, Wardruid Loti, Power of the Wild.
Don't keep Crystalizer.
Strategy: Be sure to kill Northshrine Clerics and Acolyte of Pain without letting them benefit of the extra draw potentional they can offer.

Rogue: Assume it's Miracle, can be Pogo too.
Mulligan: Wardruid Loti if they have the Coin.
Strategy: As for the date of this guide, Miracle version doesn't run Fan of Knives - be sure to take advantage of that.
If you recognize the opponent plays Pogo version be alert of Vanish.
Wardruid Loti is good to have around in case the opponent makes an Edwin VanCleef play, however it is not effective to set-it up preamtively. solid card in other forms too if it wasn't required to counter Edwin.

Shaman: Assume it's Murloc/Overload aggro version, can be slow version or midrange version too.
Mulligan: Wrath, Power of the Wild, Swipe.
Strategy: Be alert of Underbelly Angler and Thunderhead and be sure to terminate them as early as they come to play.
Let no murloc survive, as they have synergy cards.
Keep in mind cards like Bloodlust and The Storm Bringer.

Warlock: Assume it's Zoo.
Mulligan: Wrath, Swipe, Druid of the Scythe, Wardruid Loti.
Strategy: Complete board-control game. it's good to have Rush around to take the Carpet down.

Warrior: Assume it's Bomb/Fatigue.
Mulligan: The Forest's Aid.
Don't keep Crystallizer.
Strategy: Try to play around his removals and make boardstates that are difficult/expensive for him to remove.
Starting on turn 4 you could be rushed by Millitia Commander so having a high-health low attack minion is not effective.
Starting on turn 5 they could play Dyn-o-matic so be careful the option of this card swinging the board too difficult.
if a mech sticks (Dyn-o-matic, Omega Devestator, Clockwerk Goblin) he could attach Zilliax and take a value trade, so it's safer to trade mechs.
Don't be too scared of the Eternium Rover getting them some armor, just clear it and move on.

Matchups
All matchups are either relatively close, or very much in your favor, so there's a good place to be optimistic.
Warrior has to be the worse matchup, however not by far and you can still beat them many times.
I have no idea who's favored when playing against Mage or Rogue, both decks are scary and yet got a weakness to aggression.
Those 2 decks are difficult to play so if you'r playing below high legend they will not pose a threat as big.
If you meet a lot of Hunters and Paladin - those are the matchups that you are especially excited about.

Specialist
I will be providing general advices in this section, as I'm not certain myself. it will not include which cards to remove.
1: The deck can make a good primary deck as it is. can add Sea Giants to it to make the Secondary better.
2: You want to have anti-aggro list, and potentionally one you queue against rogue too.
The most important thing is to add 2 Sea Giants. you might want to play the giants in the Primary to use the secondary for certain techs only.
Additional cards that could added too: Zilliax, Hungry Crab, Batterhead, Mecharoo, Giggling Inventor.
3: The 3rd list can be used to add counters to one of the meta decks.
That could be mage, by adding Big Game Hunters, Faerie Dragon and Crazed Alchemist.
It could also be designed for warrior by adding cards such as Harrison Junes, Cairne Bloodhoof, Mojomaster Zihi.

Gameplay, Questions and self-promotion
You can watch hours of gameplay with me explaining my thoughts process in my Twitch channel, June 23-24.
I was learning the deck myself throughout the stream, and you can expect minor card swaps through it.
This is also the best place to ask me questions and get immidiate answers, however I'm still reading your comments here too.

www.twitch.tv/hatulhs