Hearthpwn

OTK Priest against Heroic Majordomo Executus/Ragnaros the Firelord!

The Gameplan

Win by setting up an unkillable wall using Deathlord or Mogu'shan Warden with priest buff spells. Do everything you can to keep your big taunt alive and take down the minions that Majordomo plays. Afterwards, get Majordomo close to 21 health and never any lower. This will prevent him from playing one of his ten Molten Giants in his deck. Eventually, his hand will fill up with all 10 of his unplayable Molten Giants and you can continue to build your big taunt into a 60/60 monster. Finish Majordomo off with whatever you like and land the final OTK on Ragnaros. Even if you must use your big taunt to kill Majordomo, Ragnaros can't play a taunt or kill you or your taunt on his turn.

Early Game

A strong opening hand has minions that you can trade against his 3/3 minions such as Dark Cultist, Mind Control Tech, Lightspawn, and Earthen Ring Farseer. Don't be afraid to play Mind Control Tech or Big Game Hunter even if their battlecries go unused! These minions also tend to soak up his precious Lava Burst and Fireball, so that your Deathlord or Mogu'shan Warden can comfortably come in (with buffs). Ideally, your taunt comes in buffed with Velen's Chosen to kill off the remaining 3/3 minions or so that the spell damage makes Holy Nova do 3 damage. If you feel the deck is struggling against the early 3/3s, feel free to try out cards that directly take care of them such as Shadow Madness, Shadow Word: Pain, Auchenai Soulpriest + Circle of Healing, or even Lightbomb.

Late Game

Sea Giant and Kel'Thuzad help reclaim the board. Sea Giant is capable of appearing very early in the game while Kel'Thuzad will let your smaller minions continue to dominate the board. Save all of your healing and buffs for your big taunt. If Majordomo brings his health below 21 with a Flame Imp, you must heal him back up to 21 health. Remember, you don't want to damage Majordomo until you have lethal on Ragnaros the following (or same) turn, so continue to sit around until your big taunt is big enough.

Edit: I've added a little extra healing in Earthen Ring Farseer and replaced a Sea Giant with Kel'Thuzad. The extra 3/3 bodies and the bonus 3 healing helps immensely when establishing the big taunt and Kel'Thuzad can really turn the game around if you have some extra bodies on the board. Sometimes you lose if he starts with most of his removal (he only has 4), but it wouldn't be fun if it were fair. Good luck!