Hearthpwn

Beat Omnotron with this deck on the first or second try.  The idea is to use your spells (frostbolt, fireball), hero power, and toxitron's own ability to kill off his minions in the early game.  Then once Electron is on the board, you try and stall Electron/mitigate damage for a couple turns (ice lance, ice barrier, cone of cold, frost nova, mirror image) until you get Gadgetzan Auctioneer out.  Once he is out, try and get a mana wyrm down.  Then just start casting spells both to the face and to his minions (don't kill Electron, just everyone else) until he goes down.  The combination of face damage from your mana wyrm and spells should easily take him down.  

I included a couple voodoo doctors in case you need them to make sure Electron doesn't die.  If you have another Gadgetzan Auctioneer, you will definitely want to put him in.  I didn't however, and this is the exact deck I beat him with.  Lastly, I didn't even use Nefarian so that one could be subbed out as well.