I've been playing Hearthstone for around 8 years now but I've never played a more competitive deck than this one. The games were player between Gold and Platinum but it has Legend potential for sure.
If anyone is interested let me know and i will also make a guide.
Short Guide (let me know if more details are needed):
In a meta in which everyone is trying to kill you fast, this whole deck is designed to postpone the opponent's win condition by lots of removal, healing, discovering the cards in your deck that you need and good card draw, usually winning in turn 10-11 by completing the quest or enemy just conceding for running out of offense.
Also I carefully chose the cards by their cost in order to be able to complete the first phase of the questline (2, 3 and 4 cost cards) rather quickly until I draw/discover the next part of the quest. Then just survive by playing the big removals/healing when you need it and ultimately Shadowreaper Anduin and Psychic Scream to complete the quest and win,
For mulligan try to keep pieces that help you complete your quest ( usually 2,3,4 costs that you can play ) , complete your quest and then just win. Wait for his board and use mass removals to kill his offense and continue pursuing your quest.
Against aggro just keep your healing resources and removals and try to survive by playing the right card at the right time. Don't overuse your removals but rather try to make full use of them
For mulligan against aggro forget about your quest for a while, try to keep your cheap cards to survive until turn 6-7 when you can clear the board and get healed. After that you cannot lose anymore as you'll notive you have answers for basically anything they throw.