Hearthpwn

First and foremost, the following matchup guides are written according to what I think is the best strategy. If you have a different approach, and it works, please share it.

Overall, because most people will be expecting cubelock, they will mulligan accordingly, so have that in mind in every matchup. To further take advantage of this, if you're playing in high legend, in Battle.net, choose the option to appear as disconnected, because this way your opponent won't be able to see your name and, therefore, identify you as "that guy with the zoo deck I played against before".

In general, try to keep the coin for powerful stuff: that means coining out 5, 7 or 8 drops, as opposed to coining out 1 drops.

A general tip for every matchup: topdecking Blazecaller on turn 7 happens more often that you would think, so playing a 5 drop + a 1 drop elemental (instead of the other 1 drops) is not a bad idea sometimes.

A final warning: in the list below, I mention what the IDEAL cards to keep are in every matchup. However, be aware that this deck is top heavy for zoo standards. For example, if you have a bunch of 1 drops in your starting hand, and you toss them all away to try to find the ideal stuff, there's a chance you draw a good part of your lategame. Mulligans with this deck are EXTREMELY important, but also very tricky.

• Warlock:
  • Mulligan: keep Kobold Librarian, Prince Keleseth, Darkshire Councilman, Spellbreaker. All other 2 drops get traded in rather easily by their Mistress of Mixtures and Kobold Librarian, so your best bet is only to play your own kobold in hopes of finding keleseth.
  • Strategy:
    • Overall:
      1. By turn 4, try to have a minion that survives Hellfire (preferably Darkshire Councilman, but can be Tar Creeper too)
      2. Play around defile the best you can
      3. Try to have a good board before turn 5. That way, if they play the weapon, they are going to be taking tons of damage.
    • Control warlock: be wary of Twisting Nether
    • Cubelock: be careful, Mountain Giant and Doomguard are no joke.
• Priest: most of the time will be combo, but sometimes spiteful. Therefore, the following mulligan is made to beat combo.
  • Mulligan: keep Flame Imp (to contest Northshire Cleric), Prince Keleseth, Darkshire Councilman (because it survives Duskbreaker, but keep it only if you have a 1 and a 2 drop), Spellbreaker.
  • Strategy:
    • Combo priest:
      1. Don't let them draw cards with Northshire Cleric.
      2. Never, and I repeat, never play a Spellbreaker for tempo or you'll regret it. Keep it to get rid of buffed minions instead.
      3. Prioritize trading, even if that means missing face damage, so that you don't get OTK'd, you'll win in the late game with Bloodreaver Gul'dan.
      4. Always play as if they had every card they need to kill you in hand, even if it's a 4 card combo.
      5. Don't throw your taunts and Glacial Shard away like crazy, use them to stop their buffed minions.
    • Spiteful priest:
      1. Early on, flood the board, and then try to have a minion that survives Duskbreaker (Tar Creeper or Darkshire Councilman).
      2. Then, try to have a huge board (preferably involving Cobalt Scalebane) before they play Spiteful Summoner.
      3. Glacial Shard and taunts will keep your big minions safe while you hit their face. I don't recommend trading at this stage, just pressure them. 
• Paladin: most of the time it will be dude, but sometimes murloc.
  • Mulligan: keep Fire Fly, Voidwalker, Prince Keleseth, Tar Creeper, Despicable Dreadlord (only if you have the coin and other good keeps, otherwise it's not worth it, as it's not that good against murlocs). The problem with Flame Imp, Kobold Librarian and Glacial Shard is that they are very bad against Argent Squire and Righteous Protector. Only keep them if you know for a fact you're up against murloc paladin. As a side note, try to keep the lowest curve possible (for example, if your hand is 2x Fire Fly, 2x Voidwalker, keep it all)
  • Strategy:
    • Dude paladin:
      1. Try to clear the board as much as you can, but don't overdo it.
      2. Try to find Despicable Dreadlord to deal with Stand Against Darkness if possible.
      3. Don't hold Spellbreaker too much, only good targets are taunts.
      4. Try to use Glacial Shard on their face or Crystal Lion.
      5. Play around Sunkeeper Tarim and Divine Favor to the best of your abilities.
      6. Remember that you always win in the late game.
    • Murloc paladin: a bit of the same, but Despicable Dreadlord is less important. Try not to leave murlocs alive.
• Mage: most of the time it will be tempo, but can be control sometimes.
  • Mulligan: always mulligan for tempo mage. That means Flame Imp (to contest Mana Wyrm, Arcanologist and Kirin Tor Mage), Prince Keleseth and Tar Creeper. Other one drops are okay. They die to the ping, but you need tempo above all things, and if they ping, they're not using their mana to do other stuff.
  • Strategy
    • Tempo mage:
      1. Be careful with taking too much damage from your own stuff (limit your taps and hold Flame Imp in the lategame).
      2. Taunts and glacial shards are your best friends.
      3. You'll never trigger Counterspell, but Explosive Runes will be your worst enemy. Try to take the least possible damage from it, even if that means sacrificing a big minion.
      4. Be careful because they usually run one copy of Potion of Polymorph, which can be tricky to play around (because of the way it interacts with Explosive Runes).
      5. Bloodreaver Gul'dan will be your salvation in this mathchup.
      6. Don't hold Spellbreaker.
    • Control mage:
      1. Vomit your hand and barely trade.
      2. Be ready to reload after Dragon's Fury.
      3. Build a massive board before turn 9 so that if they play frostlich jaina, they die.
      4. Hold Spellbreaker for Doomsayer. Can also be used in the late game to unfreeze one of your massive minions to push for lethal.
      5. Warning: silencing a Water Elemental won't remove lifesteal, as it is an aura effect. Glacial Shards are great to stop Water Elementals from healing them.
      6. Play around Meteor.
      7. Using Siphon Life in your own minions can be correct if that prevents your opponent from creating a new water elemental
• Jade druid:
  • Mulligan: keep Flame Imp, Voidwalker, Fire Fly, Kobold Librarian (only to find dr. 2 if you don't have him), Prince Keleseth, Darkshire Councilman. If you're on the coin and your hand is good, you can consider keeping Cobalt Scalebane. One drops with 1 health are not very good since they will be contested by their hero power and the first Jade Idol
  • Strategy:
    1. Play around Spreading Plague (Cobalt Scalebane is great for this) and somewhat aroundSwipe.
    2. Let them trade, you'll have to go face a lot
    3. Spellbreaker can help you ignore Jade Behemoth and Ironwood Golem (it's difficult for our deck to deal 6 damage), but play it for tempo if you have to, it's not a big deal.
    4. Try to have a tall board before they play Ultimate Infestation, so that it's more of a risk for them to play it.
• Fatigue warrior:
  • Mulligan: keep any 1 drop (but only one of them, preferably Flame Imp, for stats, or Kobold Librarian, for finding keleseth), Prince Keleseth, Darkshire Councilman.Cobalt Scalebane can be a good keep too, but only if you have an already good hand and the coin.
  • Strategy:
    1. You have so many minions that vomiting them is not that bad, in fact, sometimes you'll need to bait out their AoE to guarantee your later, bigger minions stay alive. 
    2. If you play too many small minions alongside your big minions, Brawl will cause you a lot of trouble. An ideal board would be something like double Cobalt Scalebane.
    3. Use Glacial Shard to freeze their face when they have Blood Razor or, more importantly, Shadowmourne
    4. Spellbreaker is only good against their Acolyte of Pain, so don't hold on to it too much.
• Secret hunter:
  
  • Mulligan: keep one drops that have more than 1 health (to deal better with Alleycat), Prince Keleseth and Tar Creeper.
  • Strategy:
    1. Quoting from the tempo mage guide: "Be careful with taking too much damage from your own stuff (limit your taps and hold Flame Imp in the lategame)."
    2. Fear their burst potential, but also be aware that they can't heal much, so switching to racing mode is correct sometimes.
    3. Don't trigger their secrets unless you have a good reason for it. Build a big board and then do it, so Wandering Monster becomes less of an issue and you don't give them free Eaglehorn Bow charges.
    4. Try to deny Houndmaster and Crackling Razormaw value by killing their beasts.
    5. Be ready for Greater Emerald Spellstone in turn 5. Try to have a board big enough to contest it.
    6. I'd say that you win in the late game, but once they play Deathstalker Rexxar, things can get weird.
• Kingsbane/spider rogue:
  • Mulligan: keep 1 drops that have more than 1 health (because of their hero power), Prince Keleseth and Darkshire Councilman.
  • Strategy: like hunter (because they can't heal either), but easier, because you won't fear for your life as much.
    1. Use your taunts and Glacial Shard to allow the rest of your minions to go face. Freezing is specially valuable when they have a massive Kingsbane with Leeching Poison.
    2. Avoid trading, except for Gadgetzan Auctioneer and Vilespine Slayer (only if you have tall minions, because they can use Shadowstep)
• Shaman: expect either evolve shaman or jade shaman, but don't expect it to be a well thought, optimized list, so don't be too surprised when a super aggro evolve shaman pulls out a Volcano.
  • Mulligan: any 1 drop (the bigger stats the better), Prince Keleseth, Tar Creeper, Darkshire Councilman.
  • Strategy:
    1. Play around AoE, but don't overdo it
    2. If it's evolve shaman, try to have a big board before they play Doppelgangster + Evolve, so that you can contest that.

I'll have to admit that I've barely played against these last five classes, which means this part of the guide could probably be refined. Don't worry too much though, I'd say they're the easiest ones. Enjoy!