I've finished December 2020 at Diamond 2, having played no other deck that season with a 9 stars bonus to start with. That warrants a little guide, don't you think? I'll cover the most common match ups since there's a huge difference concerning the mulligan. [Update: made it to Diamond 5 in January and February. No interest in the grind to legend, but should be possible.]

Priest: This is either Big Priest or Raza Priest, so mulligan hard for Bad Luck Albatross and Sylvanas Windrunner. Be careful with the birb though: if I have enough Mana, I usually kill it myself with Quick Shot or trigger it with Play Dead or even Feign Death to avoid it getting transformed, stolen or silenced (the latter isn't so bad as long as it dies soon thereafter and you get your hands on a Nine Lives). The good thing is, Albatross helps you in both matchups: a 5/5 birb is better than anything else they can pull out via Shadow Essence. Other than that, Carrion Studies is okay since you need to get out your stuff fast.

Warrior: Probably Pirate Warrior. Go for your secrets, Rinling's Rifle, Grievous Bite, anything that helps you control the board. If you have two solid options, you can keep Master's Call or Barnes.

Druid: Probably Aggro Token Druid, apply same strategy as against Warrior. Might be the hardest matchup together with Cube Lock. If they manage an explosive start and rather stall until they can buff their minions instead of blindly throwing them into our Explosive Traps, it's gg.

Warlock: The most popular and successful deck of this class right now is Darkglare Warlock. You should mulligan for Quick Shot and keep it to kill their Darkglare. Explosive Trap and Grievous Bite help against early wide boards. Once they get out their giants, pray for perfect draw RNG. Flark's Boom-Zooka their board, Barnes out a Scrapyard Colossus, steal giants with Sylvanas Windrunner. Be careful, they can make huge comebacks with Raise Dead. It could also be Discolock, then you're looking for the same stuff as against Warrior and Druid. Or it's Cubelock, then you lose. You have a good chance against Renolock, but I haven't seen many of those.

Mage: In depth guide for Secret Mage, the most popular list atm: We are the control deck in this matchup. No exceptions. The key to win this is that Secret Mage usually can't kill you with Fireballs and Cloud Princes alone. They need to go face. And we're good at preventing that once we've survived the early game. We should avoid procing Duplicate on their Kirin Tor Mage, Kabal Crystal Runner or Cloud Prince at any cost. Same goes for Occult Conjurer, this card is a real problem for us. Just kill smaller stuff first. Mulligan for Candleshot, secrets, Quick Shot, Grievous Bite. If you have two solid options, you can keep a value card like Master's Call, Barnes or Flark's Boom-Zooka. You can keep Rinling's Rifle as well, but be careful what you hit it with. If in doubt, go face and try to find an Explosive Trap, a Freezing Trap, a Misdirection or a Pressure Plate. Check for Counterspell before playing Nine Lives, Flark's Boom-Zooka or Jewel of N'Zoth. You can do this the turn before you want to play one of those cards. The best way to do this is by playing Carrion Studies, but I'd rather waste a Play Dead, a secret or anything else before running into that massive tempo loss of a Counterspell stopping one of your swing cards. Keep track of their secrets in general. Don't slam a low Health Deathrattle minion from Carrion Studies on the board if you suspect there's an Explosive Runes in play. Try to hit face every turn to deny them their Rigged Faire Game. You get the idea. Remember they usually don't run Ice Block.

General advice:

- Master's Call serves to fish for Barnes and Maxima Blastenheimer, but if you don't get them offered, take a Darkmoon Tonk in case you're pulling Oblivitron from Barnes or Maxima later.

- Remember that double Nine Lives with a Darkmoon Tonk in your graveyard on an empty board means 16 face damage!

- Try to get off Play Dead, Feign Death and Nine Lives as often as possible before playing Zul'jin. You also should have played at least one Jewel of N'Zoth or Flark's Boom-Zooka before doing so.

Finally, I'd like to stress that the deck probably won't work without Barnes, Maxima Blastenheimer and Flark's Boom-Zooka. This is about mana cheating your expensive Deathrattle minions out when you regularly don't have any business to do so.

Have fun!