Just a note on this deck, if you are running into a lot of midrange hunters, you want to go drop the Leper Gnomes and possibly undertakers, and put back in Power Overwhelming and Void Terror. Midrange has a hard time dealing with bigger minions as well as the 4/4 nerubians, while this deck as constructed is meant to maintain board control via deathrattle minions, it doesn't build the big minions that the Void Terrors do. This deck is much better against the cancer/secret type decks, so know your meta.
This is the deck I used to hit legend. The initial intention of this deck was to re-gain the advantage over secret/mad scientist type hunter decks. When mid-range hunter started gaining popularity, the Void Terror/Nerubian Egg synergy along with occasional chomping of minions buffed with Power Overwhelming was godly against mid-range. But when secrets hunter came out, Freezing Trap became a real issue with tempo, as well as mad scientists being able to easily take out minions such as Flame Imp. Frankly, Void Terrors are too slow to beat the mad scientist hunter deck. Once I switched out my Void Terrors and Flame Imps for Undertakers and Leper Gnomes, I quickly shot up into legendary.
You will note, this deck does NOT contain Flame Imp. This is intended. In the current meta, 2-health minions without a deathrattle are awful. Plus, the extra 3 damage to your own face can be deadly against hunters, where games can be very close on life/health as they are trying to pressure you as much as you are pressuring them.
The general strategy is to get your minions to 3 health as much as possible. Initially, this was built to defeat explosive trap. What I found is that this strategy pays off huge against other classes as well.
Starting hand mulligan strategy is a little different than you might expect. You don't always go for Undertaker and deathrattle minions. Of course, it depends on who you are playing. Here is a quick rundown of mulligan strategy:
You want to hard mulligan for Argent Squire and Voidwalker as your one drop attack minions, and also for Abusive Sergeant and Dire Wolf Alpha. You also should not mulligan your Ironbeak Owl, because this card will gain you insane tempo versus Mad Scientist when you silence it. Haunted Creeper is also worth keeping in an opening hand, assuming you can pull a Dire Wolf or an Argent Squire. Remember, this deck is built specifically to deal with hunter secrets. Your goal is to either silence the secrets, or work around them by boosting your minion's health to 3, and also using buff minions to attack first (when a secret comes out, always always attack with cards like Abusive Sergeant or Defender of Argus. Getting battlecry minions back in your hand is almost like a free shadowstep).
Hunters can still mess you up badly with Buzzard/Unleash the Skill combos, and weapon/secret synergy. But with the kind of starting hand I described, you can always handle webspinner, mad scientist, and animal companion fairly easily, gain tempo, and roll over the hunter. Ideally, you want to try to draw into Defender of Argus. Hunters only have 3 removal spells, so one voidwalker, 2 other minions, and a defender of argus = very sad hunter, even with combo.
In recent days, it seems that hunters are becoming aware of this deck. If you find hunters beating you, you want to use the trade I suggested below, -2 Leper Gnome, +1 Power Overwhelming +1 Ironbeak Owl (the extra PO is mostly for activation of the nerubian eggs for more pressure on the hunter, but also helps as a finisher)
Priest, you absolutely want argent squire + abusive sergeant. That said, if you have coin, Undertaker + leper gnome is godly. Your goal in the first 2 turns is to be able to do 3 damage on turn 2 to take out Northshire Cleric. This can also be achieved (without coin) by using Leper Gnome + Dire Wolf Alpha. Do not be afraid to aggressively kill the Cleric. A cleric that lives is absolutely deadly, and a cleric that dies will make the priest very sad. Also, often on turn 3 the priest will get out a deathlord. This is where you use things like Soulfire aggressively to remove the Deathlord.
Here's a cool tip you might not know: If you have an undertaker on the field, and you kill a deathlord, if the minion that is summoned has a deathrattle, your undertaker will be buffed.
Mulligan aggressively for undertaker and deathrattle minions. Paladins have no good way of dealing with your undertaker in the first 3 turns, so you can really put the screws to paladins with an undertaker. Also, never mulligan out abusive sergeant (sensing a pattern here?). You can even aggressively throw down a Knife Juggler on turn 2 knowing the paladin can't do jack to it.
Warlock (demon or zoo):
Argent Squire or Voidwalker + abusive or dire wolf. An argent squire with a + damage buff minion is such a huge tempo gain on any early game.
Possibly the toughest matchup. This is all about doing as much damage as soon as possible. The goal here is to get 4 damage on turn 2 to take out any early Armorsmiths. This can be achieved by leper gnome turn 1 followed by abusive sergent, or, if you have coin, something like voidwalker + argent squire turn 1, followed by a dire wolf alpha turn 2. This game you want to race.
One thing: Do not play around Brawl. You can sort of play around it by playing haunted creeper and nerubian egg aggressively, but this matchup is a race. If the warrior has a brawl, and RNGesus gives him a favorable brawl (by, say, keeping alive a 0/2 nerubian egg), you're probably going to lose. Warriors only run 1 brawl however, and by turn 5, you should be looking to get the warrior into lethal range.
Updated note: If you are running into a lot of control warriors, I would not play this deck. Warriors have too much aoe and removal, and too much healing in the late game, you're lucky to win 25% against control warrior at higher ranks.
If you are facing a beatdown warrior, you want to get your deathrattle minions on the board and buffed by defender of argus asap.
Very few shaman on the ladder, but they can be a tough matchup. For this one, I like to keep as many of my "toughie" minions in my hand as possible, argent squire, haunted creeper, harvest golem. Shaman have a lot of outs against you, so your goal is to either have minions at high health (4 health and up), or low health (1 HP). Mulligan out Nerubian eggs, because earth shock will make you sad, and it's better to combo the eggs in the later game with either undertaker or defender of argus.
I pretty much don't see druid at all on the ladder. But, if you run into druid, your goal is to sucker out wrath on either your argent squire or voidwalker, and then flood the board with undertaker and deathrattle minions. With all your buff minions, you should be easily able to kill their 4 and 6 health minions, save Owl for Ancient of War, and try to get as many minions at 2 health as possible, as very few druids run starfall, and 2 health minions will survive the ping 1 of swipe. If you are close to lethal, you can ignore Ancient of Lore, but don't let an Azure Drake live because of the swipe synergy.
Rogue is sort of an all-or-nothing type RNGfest, if you get your right draws, you'll be almost impossible to beat, and same for the rogue, however if you both draw well, the Warlock should win. Definitely mulligan again for Argent Squire, Haunted Creeper is also really good, and your abusive sergeant will help those 1 attack minions knock off any SI:7 agents or earthen ring farseers. These tough minions will then last through mid-game and pressure the face of the rogue. Not having Flame Imp makes this version of zoo way, way better against Miracle, since the extra face damage from flame imp puts you closer to the Rogue's Leeroy/Shadowstep reach lethal, and flame imp is so easily removed by backstab.
General Card notes:
Because I was facing a lot of warriors and midrange hunters, I made the following trade:
Knife Juggler - The goal with this guy is to synergize him with an attacking Haunted Creeper
Haunted Creeper - this, my friends, is the most imbalanced 2 drop in the game. Every warlock, hell, every deck, should have this card in it. It's got 7 total power for 2 mana. That's 3.5x power for mana. Even the uber value card Cairne Bloodhoof has only a 3x power for mana value. And at 2 health, it lives against so much more than you would expect, for example webspinners, argent squires, Paladin "guys," it absolutely wrecks druids who have to waste way too many resources to kill it... on and on. Basically, if you are a warlock, and you're not playing Haunted Creeper, you're doing it wrong, and I would expect that eventually most of the deckbuilder theorycrafters are going to catch on, and this thing will be in every single deck just like Pagle was when he was overpowered back in the day.
Proof of Legend:
My highest legend ranking so far this season is 208, including beating well known players such as tidesoftime and ErA.