Hearthpwn

Your general strategy is to play for board control early, to allow your minions to deal enough damage, for you to be able to rush/burn/heropower your enemy down in the mid or early lategame. By utilizing your hero power and synergies you should be able to maintain board control versus other aggro decks. In the control matchup you should try  to focus on an explosive start overall. Jeeves and Hobgoblin will help you squeeze in the finishing damage.

A common mistake is to play for the midgame from the beginning - not playing a Haunted Creeper or a Mana Wyrm early enough, because you want to get the Hobgoblin value. Do not - i repeat - Do not do this, if it weakens your turn. This is an aggro deck, which will win you a lot of games at Turn 5-7. Hobgoblin and Jeeves just enable your early game minions in the mid and early-late game. They are not your only winconditions, but instead cards, that make a super aggressive deck consistent!

I suggest you watch some gameplay videos (at the bottom of this post) to get an idea about how to play it, because it may seem counter-intuitive for some players.

• Unstable Portal is a great card on it's own, which often turns the game in your favor. Furthermore it synergizes with Mana Wyrm and Knife Juggler (It is a spell and a Minion)

• Clockwork Gnome is a perfect one drop for this game. It is superior to Leper Gnome in every situation, unless you push for the final damage. It synergizes with Undertaker and by giving you a Spare Part it synergizes with Mana Wyrm as well.

• Echoing Ooze is best used with Hobgoblin [/card] for great board advantage. If you are in need for a board (Most of the times vs. Hunter or Paladin) or need to squeesh in some more damage to be able to utilize your burn spells, you can play him blank or in synergy with [card]Knife Juggler , too.

• Haunted Creeper is sticky and annoying. It synergizes with Undertaker, Knife Juggler andHobgoblin, which makes it a perfect allround card for this deck.
  
  
  
• Mirror Image synergizes with Knife Juggler and Mana Wyrm. Additionally it protects your board in the control-matchup. Allowing you to do the damage your creatures have to do, before you overwhelm/burn the opponent. Against aggro it helps you to establish board control and get the superior trades.
  
  
• Fireball & Frostbolt gives you additional reach for a total of 18 Damage burn. Keep in mind how much burn damage you have left in your deck at all time. It allows you to play risky moves, relying on a Topdeck / Jeeves grab. It is totally fine to Fireball a Sludge Belcher to protect your board most of the time!
  
  
• Counterspell is debateable. The purpose of this spell is to prevent a boardwipe with Brawl,Flamestrike,Swipe, Consecration, Holy Nova, Shadowflame, Unleash the Hounds etc.. I would say it works in 50% of all times - in the other 50% this card just sucks and even hinders your gameplay if you draw it instead of being able to activate it with Mad Scientist. Play it depending on the meta you face.

General Mulligan rules:

You always want to have something to play the first 3 turns, which means you should drop everything, which costs 3 or 4 Mana regardless of the other cards in your hand.

• If you are going first, your main goal is to get a 1-Drop to establish boardcontrol, which means you should go for the 1 drop every single time, if you do not already have one. Your chance of getting at least 1-Drop (not including Mirror Image) at the start is 29,6%.
  Your chances of getting a 1-Drop by mulliganing your whole hand, while going first is 76,7%. Which means you should have something to play on turn 1 in 86% of the games, if going first.

• If you are going second, you will be able to plan your turns ahead a bit more. If you have a good 2-Drop or some synergy on hand (e.g. Knife Juggler + Mirror Image) just keep them and mulligan away all the other non-fitting cards. Your goal is to have something to play and apply pressure, not to have the perfect opening every single time.

• If you are having an Undertaker in your Hand it is advisable to go for the Undertaker dream start and mulligan everything, which does not have a deathrattle or does not fit the curve. Even if Undertaker gets removed, it will cost your opponent a turn, allowing you to swarm the board even more – applying pressure.
  
  
• If you are having a Mana Wyrm and you are going second, you should definitely keep it (combining it with the coin) and look for some synergy with Mirror Image or Unstable Portal.

Playing versus Zoo, Hunter, Rogue & Mechmage - Exact timestamps can be found in the youtube description:

 Playing versus Paladin, Hunter, Shaman & Zoolock (Last game is memorable - Never give up!) - exact timestamps can be found in the youtube description

Playing vs. Hunter (2x) and Handlock (2x) - exact timestamps can be found in the youtube description

For all the YouTube - Lovers, that prefer watching instead of reading:
Please notice, that the provided content is not from me, but from RubenVDB 
I would not mulligan for Hobgoblin, but he succeeded with it and gives you a good insight in the decks mechanics:

The amount of feedback through comments and personal messages is really overwhelming. Thanks for all your great ideas. To keep it short and simple i decided to write some answers to cards, that can be found in the comments as a suggestion/hint/idea.

But first of all: I am not sure how most things will work out in the long run, because i made legend with the deck i already posted. The best thing to test an idea, is to try it, which should be really easy because this deck is not really expensive.
Furthermore: My opinion is just an opinion, not a fact or something to rely on ;-) If you are able to evolve an even better deck - do it. Hearthstone is a nice community and evolving decks together is fun.

• Annoy-o-Tron could work really well, because it synergizes with Hobgoblin. Sadly it has no deathrattle, but it is annoying as hell for most opponents, giving you some time to get the damage in with your early game minions. Definitely worth testing.
  
  
• Micro Machine needs immediate response from your opponent and synergizes with Hobgoblin. It could definitely work in this deck, especially if you face a lot of Control.

• Echo of Medivh is a great card, but i think it is too slow for this deck. It cripples your hand in the early game and counters the draw from Jeeves in the Midgame. If you really want to play Echo i would suggest to cut 1 Jeeves and do some testruns.
  
  
• Novice Engineer is suggested by a lot of people to replace Loot Hoarder, because it synergizes with Hobgoblin. I prefer the additional Deathrattle from Loot Hoarder, because i face more situations in which i want to buff an Undertaker, than situations in which i do not find enough minions to combine with Hobgoblin. Considering this and the additional +1 Attack makes Loot Hoarder a more early game card, than Novice Engineer is, which is why i prefer Loot Hoarders. 
  
  
• Enhance-o Mechano could work as a finisher, but i am not really convinced. With 4 Mana this card costs a lot and stacks your hand early, which sucks if you have to draw with Jeeves. It is a good card to draw, if you are already winning, but it can hinder your gameplay and aggression if drawn early. I would be extremely carefully changing the curve.
  
  
• Acolyte of Pain & Flying Machine are really slow, but great value cards if they are combined with Hobgoblin. Sadly this is not a value deck, that relys soley on Hobgoblin. It is an aggro deck, that transitions surprisingly well into the mid- and early lategame, because it relys on a lot of cards, that are strong on their own (and Jeeves as well as Hobgoblin). If you center the deck or your gameplay too much around Hobgoblin, you won't win consistently. 
  Just add them, if you want to play gimmicky and do not plan to climb.