Hearthpwn

I can't figure out any other way to beat him.

It is a pretty hard boss and you'll need A LOT of heals. You not  only will need good mills, but you also need good heal turns. Turn 6 fish + prep vanish, or some fish prep vanish before turn 6 is a must to win.

This guy will randomly hit you for 10+ damage and have those big ogres that you need to deal with early. You'll need more than 2 Vanish board clears, so use Blade Flurry for extra clears. If you leave his board full for 1 turn you'll lose because you can't afford to take any extra  damage.

The early game is the hardest part and where you need your cards to align. If you get that good early game(up to turn 6), you should have drawn enough cards to make good plays. Be aware of that Boulderfist Ogre + Charge + those crazy damage spells he has. Always exepct some 8 attack minion charging your face. Also, because of his armor, armor gain and hp, even with 2x Gang Up you'll take some fatigue damage, so expect to take 6~10 fatigue damage yourself.