Hearthpwn

Generally you want Plated Beetle in any match-up. Its great for surviving early and provides average board presence. The below mulligans are ranked, try to get at least one of the top 3, if you only have a 4 or a 5, hard mulligan the whole hand.

• Mulligans for Paladin: 1. Mistress of Mixtures 2.Plated Beetle, 3.Doomsayer 4.Stubborn Gastropod
• Mulligans for Mage Hunter Shaman Rogue Druid: 1.Plated Beetle 2.Mistress of Mixtures, 3.Doomsayer 4.Primordial Glyph
• Mulligans for Priest: 1.Stubborn Gastropod 2.Primordial Glyph 3.Plated Beetle, 4.Frost Lich Jaina, Special case: If you are going second or you have 2 of the 2 cost cards 5.Polymorph
• Mulligans for Warlock: 1.Plated Beetle 2.Primordial Glyph 3.Doomsayer, 4.Coldlight Oracle, Special case: If you are going second or you have 2 of the 2 cost cards 5.Polymorph. Special Case 2: If you are only ever seeing control and no zoolock, feel free to keep 1 or even 2 coldlight oracles
• Mulligans for Warrior: 1.Plated Beetle 2.Primordial Glyph 3.Doomsayer, 4.Stonehill Defender

Further Strategy

If you have a 2 of your key 2 cost minions, you can keep one of the following if it is strong in that match-up: Stonehill, Polymorph, Coldlight, Jaina, Dirty Rat

• Turn 1 or 2?! Almost never. BUT you need to know this. No self respecting hearthstone player will keep a 5+ mana non-combo creature in their hand. So keep a close eye on how many cards they mulligan. The less cards they mulligan the more playable Dirty Rat. If they keep their whole hand and you are worried about just 1 or a few cards (e.g. Hunter's Y'Shaarj, Rage Unbound) then you are pretty safe. Against Aggro Mage and Aggro Paly it can be beneficial to grab most of their starting cards, just know that the chance of you pulling out a danger card like Kabal Crystal Runner or a Crystal Lion can quickly exceed 50% if there are 3+ danger cards and they mulligan 3+ cards.
• To combo out Doomsayer+Frost Nova. Help protect Doomsayer by grabbing a silence, filling their board or simply providing a taunt to help stop any combo removal
• Quest mage. Wait until just before they are about to play out their combo for maximum chance of pulling combo cards
• Aggro Mage. ASAP if they don't mulligan (read point 1). Otherwise wait until 1 or both Kabal Runners have been played or you think they have no creatures in hand. Also, you can use it in a combo to clear Explosive Runes
• Aggro Paladin and most other aggro. Pretty much as above but usually used more to combo out Doomsayer or disrupt their combo cards, E.g. Sunkeeper Tarim Lightfused Stegodon Gentle Megasaur
• All Control/Combo Decks. Try to wait till you have the best chance of pulling out key cards.  Look for signs of people setting up a card, e.g. you will see players clearing their board to set up N'Zoth, the Corruptor

1. Against combo or control. Pretty much whenever it is reasonable to play and you see them with 8, 9 or 10 cards in their hand. Special cases:
   • Control Warlock. Delay playing till turn 4, if you play on turn 3 you will just make his mountain Giant playable
   • Druids that play Ultimate Infestation. When it looks like he is setting up to play UI
   • Mill yourself? Sometimes if you already have your key cards and you think it is your best chance of winning, milling one card to mill them 2 or 3 is reasonable. That said, you are better off setting this turn up so you don't mill yourself
2. Against aggro. Sometimes it can be a dead card in hand, there are really only 3 playable scenarios:
   • When you are trying to draw into a card just to survive or set up Doomsayer+Freeze combo
   • When it would benefit you more than them, similar to above but you aren't in immediate danger. This one takes practice
   • When you are semi-stable and want to bring them to fatigue faster

Recommended Removals

1. Dirty Rat can be hit and miss, playing just 1 is fine
2. Stonehill Defender 2 taunts for the price of 1. But but plenty of decent replacements
3. Stubborn Gastropod strong against turn 1. one attack minions and can draw removal to set up doomsayer. Can be hit and miss though
4. Arcane Intellect

1. Disruptive secrets. Counterspell and/or Potion of Polymorph are feasible
2. Taunt replacements. Tar Creeper, Lone Champion, Second-Rate Bruiser
3. Untested but may be playable: Arcane Tyrant, Elise the Trailblazer

Tested and Not Recommended

• Babbling Book or Yogg. FUN but then... RNG
• Pyros generally too weak and too expensive
• Other big cards, Alanna, Sindragosa, Lich King, Alextrasa, Ysera. Feel free to test but I found Medivh and Baron Geddon the strongest and this deck struggles to support more big cards
• More elementals or direct damage. Doesn't fit the play-style
• Weapon/Secret removal, silence, MCTech, Kabal Courier. Too weak and situational

Hilarious game where everything went wrong for my opponent. Slight variation running secrets: https://hsreplay.net/replay/6fst5P9xAYLyJSvEJrbWBE