EDIT: There's a lot of new cards since I've updated this. Most of the cards in here are interchangeable, so feel free to swap out spells for other ones. What's important is Gadgetzan Auctioneer and mana acceleration (Innervate, Wild Growth, Grove Tender). At this point most of you don't have Grove Tender probably, but that's okay. Just throw in all the new mana acceleration like Mire Keeper and Jade Blossom. Jade Idol also makes this strategy even better, so throw that in. I'm not going to change the deck list at this point, because people aren't using this much anymore and it's like a record for me of how I beat it, but it should be pretty straightforward for you guys to make the right substitutions because the basic idea is pretty simple.
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This is the deck I used to beat Heroic Omnotron. It took me two tries, but only because I forgot to give myself a mana crystal before turn 5 the first time... The deck works by abusing Electron. He will summon it on turn 4, and if you have given yourself a mana crystal using either Wild Growth or Grove Tender (or you could just Innervate, but that is best saved) you can summon a Gadgetzan Auctioneer. Grove Tender is better for this, since it will bait out his control spells, and it won't give you draw power on turn 5. All spells cost 3 or less in this deck, which means they will all be free with an Auctioneer out. Just keep casting and you will draw your entire deck (Just be careful you don't draw yourself to death). This is basically a two-turn kill. You kill his minions (but save Electron for last or else your combo ends), probably use the first Power of the Wild to get out a panther, summon both Echoing Oozes (this is why you should save the innervates), and then spread the buffs between them. You don't want to put all the buffs on one minion, because he has some silences. Also, don't cast Savage Roar. On your next turn, you should have at least 4 buffed minions. Savage Roar twice for the win.
The most important thing is to survive until turn 5. Use your hero power the first few turns, and just try to get all his minions to 1 HP. That way, either Toxitron will kill them for you, or he'll use Earth Shock on Toxitron. And if he uses an Earth Shock on Toxitron, he can't use it on Electron. Some people have said he silences Electron, but I have never been able to reproduce it, because I always leave his minions at one health. Don't forget to give yourself an extra mana crystal before turn 5, but don't play any spells besides Wild Growth. You need to save them for the combo or else you may not be able to draw your entire deck if your hand clogs up with the 5 minions.
Subs: You need pretty much all of these cards for the combo to work. All Druid spells that cost 3 or less are included. Grove Tender is also pretty important , but you could make do with just Wild Growth. If you don't have Grove Tender or Savagery, just throw in some Wisps. They will give you additional targets for Savage Roar. Or, you could switch out both Savagery and Echoing Ooze for Young Dragonhawk and Swipe. If you are missing both Savagery and Grove Tender, put in a low-cost charge minion or two for extra turn 6 damage. The deck needs all of the mana for the combo, so you can't really afford to put in any minions/spells that are more expensive.
NOTE: Do not play Grove Tender on turn 3. He also gets a mana crystal when you play Grove Tender, so if you play it on turn 3, he plays Electron a turn early. You have to wait until turn 4 to play it.
https://youtu.be/XpxGuCBhjQ8 - Here is a playthrough I did with commentary
https://youtu.be/l4MGld44uBM - Here is boneschaman's video. His combo stalls out because his only option is to kill Electron, but it is still enough to win on the next turn.
https://youtu.be/lBGlz1mYCLE - This is Kizaru1's video. He manages to get two auctioneers out.
EDIT: Following advice from sapel, I've replaced the Anodized Robo Cub with Echoing Ooze, since they double the power of your buffs. If you also added in Wisps, be careful about filling up your board.
TGT Update: We now have another low-cost Druid spell: Living Roots. If you don't have Savagery, then definitely put in Living Roots instead. If you do have Savagery, you might want to take out an Echoing Ooze or Healing Touch instead. There isn't enough board space to play two Echoing Oozes and two Living Roots for the minions.
LOE Update: Yet another low-cost Druid spell with Raven Idol. I would definitely add this (take out Savagery/Healing Touch/Moonfire), since it gives you another chance to get Wild Growth/Innervate. Always use it to discover a spell. You are more likely to get mana acceleration that way.
P.S.: This deck does have an RNG aspect because you need to hit your combo. However, if you have 2 Auctioneers and 2 Grove Tenders, those chances are good. I did a quick Monte Carlo simulation and these are the chances of hitting the combo with
- 2 Auctioneers , 2 Grove Tenders - 45%
- 2 Auctioneers , 1 Grove Tenders - 43%
- 2 Auctioneers , 0 Grove Tenders - 40%
- 1 Auctioneers , 2 Grove Tenders - 30%
- 1 Auctioneers , 1 Grove Tenders - 26%
- 1 Auctioneers , 0 Grove Tenders - 24%