Hearthpwn

As usual with control Priest, very aggressive decks can overwhelm it unless you have a favorable draw. Always mulligan for Sphere of Sapience, Barnes, and Archmage Vargoth (nearly always performs like a 2/6 Taunt minion, plus it's the lowest-cost minion you can put on the board). Renew and Spirit Lash, and Shadow Visions are nearly always worth keeping as well. I will also typically keep any Convincing Infiltrators, but sometimes they drop onto the board too late even on curve.

What helps is that even aggro decks will often pause for a turn or two to prevent you from getting additional "free" activations of your quest. That breathing room can make the difference.