I played this deck from D5 to legend.
counters warrior, druid, DH and -if you get good hand- HLhunter
thats like 80% of matchups so ez legend.
pd: I don't have the other priest legendaries so i had to make this deck.
+10 = guide
mulligan and playstyle
*always keep Disciple of Galakrond
Try to Shadow Madness the bomber girl and zixor. If, bomber warrior then just use Gala and tap every turn to get 10 cards in hand and burn the bombs when draw.
Use Penance on the freeze girl, hero power face every turn if possible.
When you have Acidic Swamp Ooze in hand and you are getting to turn 5 just heal face and remove something, so he plays naked weapon and then you Ooze him.
Don't play Gala too early just keep healing face, play Gala if: you need the 5 armor, need to remove something big or you are out of resources.
Try to count your opp portals/board fillers and use your aoe removals accordingly.
Druid is easy if he doesn't get 3 Ysera, Unleashed from the "6-mana discover ysera" (yes, it happened to me)
- Warlock: Just play in curve and save all your healing and 1 big removal in case your opp plays naked Alexstrasza
- Mage: Curve then lose by rng. Try to Shadow Madness Astromancer Solarian. Use your removals carefully go for value trades if possible. Use Gala to get more resources.
- Priest: Galakrond, the Unspeakable pack, tap every turn if possible, go for value trades, try to save some heal or a big tempo swing combo to beat late game/fatigue.
Try to steal the Murgur Murgurgle and remove/value trade everything so he can't buff.
- Shaman: Breath of the Infinite, GAL cards. I didn't played against any shaman.
- Rogue: Shadow Word: Death, GAL cards. Try to get ahead on board since rogue doesn't have removal spells. Save Plague of Death for rogues big tempo swings. This is the hardest matchup.
I'll answer your questions