Clearly I don't know how to priest. Any tips on priest in the dungeon would be awesome. I feel like I've tried it all at this point. Double battle + all cost 5 + dragon or c'thun seem to get me to the end but no matter what, the decks get crushed by the final boss. Regardless of who it is.
I've completed it 3 times, once with a 2xPotion of Vitality Reno deck, once with a C'Thun+double Battlecry+Enemy minions cost 1 more and yesterday with a relatively bad Reno deck with 2 Rags (Chose Potion of Vitality and Enemy minions cost 1 more). The C'Thun was the easiest out of all, but making enemy minions cost more is really powerful as well. You might want to concede and restart if you don't get a good Passive after the first boss. Of course there's a lot based on RNG when you come to the last boss. For example, I've never lost to Xol but beat The Darkness only once out of like 10 attempts (with gigantic Jades).
Priest turned out to be one of my easiest classes to do the dungeon run with, I think I got a bit lucky with my deck selection though. Maybe you can copy some of what made it so good into your own deck.
I had Quest Priest with Wax Rager and Potion and Vitality. I think Quest Priest is pretty viable in the dungeon run so if you get a chance to pick it early, I would say go for it. Wax Rager is of course OP so if you get a chance, it's probably a good idea to take that card.
I wish I could remember more about the deck, I did the run a month or more ago. The real key is just to keep trying and take time in every decision. They all add up and they all matter.
Rogue was the hardest for me, I had over 100 bosses defeated before I finally got the win! Just keep trying, the game will give you some good s**t eventually.