Hearthpwn

the deck i used for Thaddius on heroic. thaddius starting off with stallagg - 7/4 and feugen - 4/7 is pretty daunting but honestly, I feel he's one of the easier bosses...if you are willing to restart your opening hand.

for your opening hand, up to turn 2, you absolutely need an Equality. this will make the monsters go to 1 health on turn 2, so on turn 3 their attacks will be 1.
second, you need Unstable Ghoul by turn 3. why unstable ghoul? it deals 1 damage, and if you put it down turn 3 after equality was used turn 2, on turn 4, the monsters will revert back to 1 health and when ghoul dies, boom, so do they.

once you take them out, the fight becomes significantly easier, though by no means is it a cake walk. from here, you want to summon high cost minions that can trade and deal nice damage.  outlast him, heal, deal damage, and after a few attempts, you should do all right)

tech choices:

Nerubian Egg is used because when it's stats are flipped it becomes a 0 health minion so it dies and you auto get a 4/4 Nerubian. gameplay mechanics are fun huh?

updating a 2 year old boss deck?  why yes i am.  just putting in Stalagg and Feugen due to, when the bosses stallagg and feugen die, yours will instantly summon thadius.