Heyo, I used this deck to climb to legend for June 2020. (I also used Quest Warlock, but with only a 48% WR). So I guess it would be "Bronze 10 --> Legend".
To keep things nice and short, this is a typical Galakrond Rogue using the stealth package and a Malygos OTK. Last month, I tried to run a similar list without the secret package, and also using Umbral Skulker. However, this earlier iteration was running into problems with late game card draw and assembling the combo consistently, so I added the powerhouse Greyheart Sage.
The overall plan of the deck is, as you probably already know, is to combo-kill using Malygos and Sinister Strike(s) and/or Eviscerate(s). While the early game plays the same as any other Stealth Galakrond Rogue, the late game typically revolves around assembling the combo through cost reduction effects (including the incredibly rare use of Escaped Manasaber for an extra mana). It is useful to count the damage in your hand often, as you might be sitting on lethal without Malygos and not even notice.
Flex Slots: Preparation, Sap, and Bone Wraith.
Always keep Greyheart Sage!
Usually mulligan away Sinister Strike, Preparation, Kronx Dragonhoof, Malygos, and Heistbaron Togwaggle.
Demon Hunter: Want Backstab, Seal Fate, and Spymistress.
Druid: Want Edwin VanCleef, Escaped Manasaber, and Flik Skyshiv (for Glowfly Swarm).
Hunter: Backstab, Seal Fate, Devoted Maniac and Shield of Galakrond. Pray that it's Highlander and not Face.
Mage: Skyvateer, Shield of Galakrond, and Galakrond, the Nightmare. (Just want value :D)
Paladin: Skyvateer, Seal Fate, and Sap (for Libram'd Minions).
Priest: Same as Mage^. Play for value and OTK. This is the deck's best matchup IMO.
Rogue: Also very similar to Mage and Priest^^. I haven't played against many Malygos Rogue lists in ranked, so the mirror match is in your favor with the OTK potential of this list. Flik Skyshiv's battlecry will hit your copies as well as the opponents.
Shaman: Want Flik Skyshiv, Skyvateer, Seal Fate, and Shield of Galakrond.
Warlock: Skyvateer, Escaped Manasaber, and Shield of Galakrond to establish a board presence, but be careful not to overextend. If you're against QuestLock, your Malygos OTK is faster and easier to assemble.
Warrior: This is the worst match-up in my opinion. Luckily, I have seen a decline in Wrenchcalibers played against me recently. Want to play a mid-range gameplan benching off Greyheart Sages, Flik Skyshiv, and Invoke triggers. Backstab is underwhelming.
Shameless Plug Time:
Find me streaming at twitch.tv/jellybadge
7/15 Edits: F's in the chat for recent nerfs. Galakrond draw was pretty overpowered (and the nerfs were definitely fair) but wow did they hurt this list. Because of the inconsistent OTK potential when Malygos is not discounted, as spells will cost 1 instead of 0, I replaced Bone Wraith with a copy of Umbral Skulker to enable more 25+ damage turns.