My take on Galakrond Priest without going Highlander since there are quite a few good cards in Priest that you'd want two of. The deck runs heavy on removal and a few board clears, making it quite strong against most Aggro decks provided you get atleast one of your board clears, for which the Cleric of Scales is useful.

Besides Aggro, I've also faced Token and Embigged Druid, Spell Mage, Galakrond Warlock and have defeated these decks reasonbly well. It has no card draw other than Kronx as you're getting most of your cards from either your Galakrond Invokes, Skeletal Dragon or Madame Lazul. As a result, this deck will nearly always win the fatigue battle and it is a very valid option to take it as slow as possible.

Against Aggro, try and save your Breath of the Infinite and Holy Nova until you can kill several minions with it. Just killing two 1/1's or something in that sense is too early. Wait for them to commit a bit more.

Similarly, use your various removals as the situation demands.
If your opponent plays a big minion that you can kill with Aeon Reaver, use Aeon Reaver and not say Time Rip. Aeon Reaver will only kill against minions with Attack equal or higher than their health, so don't use Time Rip on that if you can use Aeon Reaver.

Your Plague of Death should be kept in the hand as long as possible. It is your garantueed board nuke without any Deathrattles or other effects triggering and should be saved until you really need it.

Penance, Dragonmaw Sentinel and Apotheosis are there to extend your life total and to try and keep it high enough so you can't get combo'd easily. They also work well against Aggro. Siamat (with Rush) in particular works very well with Apotheosis if you have 10 mana. Rush + Windfury can be an option to gain you 16 health.

Madame Lazul is there to scout your opponent's hand to see what they can throw at you, as well as possibly get a very useful card. This can be another board clear, removal or just a beefy minion. She's pretty much always fine to throw down and see what your opponent has.

I try to save Kronx Dragonhoof until I have Galakrond active, but if you have Invoked several times and Galakrond will kill 4 minions upon playing, it's fine to throw down Kronx so you can use Galakrond next turn and destroy the opponent's board.

Soul Mirror is a card I am considering and should I add it to the deck, I will update accordingly.

This deck is entirely Control Priest and should be play as such, slow and steady and win in fatigue if you have to.