I don't feel like playing freeze mage against every heroic boss, so this is the druid deck I used to defeat heroic Thaddius in the Construct Quarter of Naxxramas.
He automatically plays Stalagg and Feugen on the first turn for free, except his versions do not have their deathrattles. When you kill them they still count towards summoning your own Thaddius so I include both myself. Dealing with those two early is very important to minimise damage taken so I took a number of cards to do this as quickly as possible: Big Game Hunter, Naturalize, and Innervate into any of Swipe, Leeroy Jenkins, Poison Seeds, or Bite will do the trick. I have also been able to get lucky with a Mind Control Tech after he played a Haunted Creeper.
He has a few nuisance cards. He runs mostly overload shaman removal in Lightning Bolt and Lightning Storm, and has at least three copies of Sludge Belcher. I loaded up on silence minions to deal with these as well as Faceless Manipulators to copy them for a 1-for-1 as well as being able to copy stuff like Stalagg + Feugen (yours or theirs!). The Faceless could straight be replaced for Druid of the Claw or your own Sludge Belchers for a more budget version.