This is the deck i reached legend with this month. It's Psygunther's list i found on Dog's stream. The only change i have made due to matchups is replacing Medivh with Volcanic Potion due to the amount of hunter/murloc midrange pala/shaman that i encountered. I really recommend switching Medivh back if u encounter a lot of mirrors or control/quest warrior.
Edit 1: Eater of Secrets can be a viable tech choice against multiple mirrors & paladin getaway kodo, but i do not recommend it since the purpose of the deck is to be always ahead/have board control.
Edit 2: Eater of Secrets is a viable tech choice. Especially after the rise of murloc paladin and Eye for an Eye , rise of Secret Mages and mages in general as well as hunters starting to tech Explosive trap vs aggro stuff like druid/pirate.
Edit 3: -Polymorph ( too slow, clunky card ) + Eater of secrets
-Volcanic potion ( i seem to be lucky and i get aoe removal from kabal or the other cards + Medivh ( value master - do not be afraid to atiesh face tank if really needed )
Proof of legend : Final boss
Mulligan guide : ARCANOLOGIST IS GOD ( very good cycle and helps later when u fish for lethal with arcane intellect so you dont draw your secrets ) , Babbling Book, Mana Wyrm, Medivh Valet, Frostbolt ( vs pirate / hunter / murloc pala )
Quick tips : Ice Block > Ice Barrier always because of Medivh Valet
Do not be afraid to play Medivh Valet on turn 2 vs aggro, board control is super important and yes Block on 3 and Valet + Ping on 4 is a play, but sometimes u need the tempo. Think twice before you make this play, but take it into account.
Use fireball even if u have to get rid of a 2-3 minion vs hunter/murloc pala if u have minions on board, i cannot stress over the importance of board control while piloting this deck. I very rarely need Alexstraza offensively to win ( mostly in tough control games ) and more commonly use it to heal myself on 1 hp with iceblock up to gain tempo and save my block.
Meteor is Semi GOD in this deck, so many people do not play around it. If u are behind, think ur trades and pings to get a good meteor. Also, use Meteor even for 1 minion if u have minions on board to keep control.
BOARD CONTROL IS KING.