Update - 5/11/20 -  (Note - I've also merged the prior updates into this one to clean up the guide)

Although I was originally dismayed at the nerf to Kael'thas Sunstrider, I've integrated the quest better in this version and believe it's still a really fun combo deck that's ladder-viable in the lower and middle ranks.  Exhibit A - I started this season with a x8 star bonus and took this list from Bronze 10 to Diamond 10 over the last 4 days.

Rationale for the Deck

• A higher-cost Kael'thas needs coins from a Quest package to be competitive
  • Result - replaced some of the early game (i.e. Dwarven Sharpshooter) and the old activator (i.e. Rapid Fire) with a basic quest package
       
• A minimal quest package (just quest and coin/draw cards) performed worse than making it an integral win condition
  • Result - replaced the emphasis on buffing and copying Zixor, Apex Predator (i.e. Ramkahen Wildtamer and Scrap Shot) with more quest support (i.e. Swarm of Locusts and Shu'ma)
         
• Added in more early survivability based on my experience on the ladder 
  • Added back in Explosive Trap and a 2nd Doomsayer to survive the early game against Demon Hunter and Face Hunter; I also found that including Khartut Defender as a 2-of improved my record against Demon Hunter in particular

While this build doesn't result in buffing Zixor consistently and using him as a primary win condition (by copying), he can still win the game with careful management of Diving Gryphon and Scavenger's Ingenuity.  On the plus side, the deck now completes the Quest much more consistently so even an unbuffed Zixor Prime can clear the way as 6/4s that become 8-attack threats if not killed.

Your main win conditions will be overwhelming pressure via Quest completion, an Unleash the Hounds finisher after Quest completion, or overwhelming pressure/face damage from Kael'thas + Nagrand Slam. 

• Nagrand Slam will also win you some games when played straight up for 10-mana because your opponent spent all of their mana/board clearing a Shu'ma or Swarm of Locusts you played/buffed the previous turn.

Mulligan

Against faster decks (e.g. Demon Hunter, Face Hunter, some of the current Warrior decks, etc.)

• Mulligan hard for your survivability cards - Doomsayer and Explosive Trap
     
• Against Demon Hunter, I'll also look hard for Khartut Defender as mentioned above

Against (relatively) slower decks (e.g. Mage, Piest, and Rogue)

•  If I'm going 2nd, I'll often shoot for the moon and hard mulligan for Kael'thas Sunstrider
  • Because you start with a coin, you only need to draw Kael'thas + 1x Licensed Adventurer + 1x Nagrand Slam by Turn 5 to pull off the swing turn that the whole deck is built around!  Totally "Wharf It!" in my humble opinion.  Tracking is your friend in this endeavor.
       
• Otherwise, I'm looking for Questing Explorer, Diving Gryphon and Scavenger's Ingenuity

Other Tips

• The 1x Marked Shot is included as a utility card.  You'll often be looking for another copy of one of the spells in your deck (i.e. Tracking, Scavenger's Ingenuity, Unleash the Hounds, Swarm of Locusts, Nagrand Slam), but you can also find value in Scrap Shot (for Zixor Prime), Rapid Fire (for activating Kael'thas), Kill Command (for lethal), etc.
      
• Face Hunter has been my worst match-up, so don't get too discouraged if they steamroll you!  You'll make it up against Highlander Hunter, Mage, Warlock, Priest, etc.

Potential Alterations

There are times that I'd still like to get a Bonechewer Brawler into the deck for the Face Hunter match-up in particular.  And being able to buff Zixor with a Scrap Shot would be nice too.

However, you could also go with a major change that I found success with last season - replacing the Kael'thas package with a Beastmaster Leoroxx + King Krush one.  I still think this is a fun + viable way to go and you'd replace the current list with something like this:

-2x Licensed Adventurer
-1x Marked Shot
-1x Kael'thas Sunstrider
-2x Nagrand Slam

+2x Ramkahen Wildtamer
+2x Scrap Shot
+1x Beastmaster Leoroxx
+1x King Krush

Or you could just gut the Quest package and go back to the shell in the original deck below...

Stay combo hunting my friends!

Hi Hunters,

I've been having fun in the lower ranks with this Combo deck and wanted to share for anyone trying to use the new cards while still being semi-competitive. 

For whatever it's worth, I started this season with a x8 Star bonus (hung out at the Rank 10 floor) and went from Bronze 5 to Platinum 10 with this list over the last 2 days.

Overall Gameplan

1. Control the board early via Dwarven Sharpshooter, Explosive Trap, Bonechewer Brawler, and Rapid Fire (only if you need to)
2. Draw Zixor, Apex Predator in the midgame via Diving Gryphon and Scavenger's Ingenuity (there are only 3 beasts in the deck)
3. Draw, buff, and copy Zixor Prime by Turn 8 via Diving Gryphon, Scavenger's Ingenuity, Ramkahen Wildtamer, and Scrap Shot 
4. On Turn 8 take over the board and establish unrelenting pressure either through (the 1st) Zixor Prime or Nagrand Slam via Kael'thas Sunstrider + Rapid Fire / Tracking / a 1-cost spell discovered through Marked Shot.

Mulligan Targets

Almost always kept:

Diving Gryphon - In almost every situation I'm comfortable playing this on Turn 3 just for the draw (i.e. even without a good target to trade)

Scavenger's Ingenuity - I keep this unless I'm playing against overwhelming aggro (e.g. DH before the 1st nerfs); you lose tempo, but because it's guaranteed to hit a 3-cost Rush minion you can usually gain it right back (unless they're going really wide and ignoring your board a la Token Druid)

Kept against aggro:

Dwarven Sharpshooter - great at delaying early development plans, especially if you're going 1st; One of the exceptions is when I'm going second DH; then I'll mullgian Sharpshooter to try to find either of the next 2 cards because they're better counters to DH playing Battlefiend on Turn 1.

Explosive Trap - one of our few AOE options

Bonechewer Brawler - pesky 2-drop that you can target with Rapid Fire if you need a little more damage to trade

Kept against control:

I mulligan almost exclusively for Diving Gryphon and Scavenger's Ingenuity, but I'll also keep Tracking against something like Rezz Priest because you really need to find your combo pieces fast before their graveyard is too OP.

Other Tips

Remember that you don't need to play Kael'thas Sunstrider before you play your first 2 spells to get its effect.  So if you only have 1 copy of Tracking on Turn 8, you can play it to search for another 1-cost spell before you decide whether to play Kael'thas (and then if you find a spell you'll still be able to combo with Kael'thas that turn).

Speaking of Tracking, you can use it more liberally than in most decks because there's a lot of redundancy that you're happy to skip over.  For example, you honestly only need to find 1 buff for Zixor Prime before it becomes tough for your opponent to clear and threatens lethal the following turn. 

The redundancy in the deck also means that you should never be too bummed about buffing or copying Diving Gryphon.  There have been a number of Turn 5s where I've played a 7/4 Rush behind a 2/3 Taunt and that's when the game starts to turn my way.

And speaking of Zixor Prime, in many games you won't need to wait a turn to copy it to ensure victory.  However, if you're playing against classes/archetypes that regularly run massive board clears then it's usually a good idea to wait until you can copy unless you're facing lethal on board (otherwise you can run out of steam against Galakrond Warlock, Rezz Priest, highlander decks that haven't used Zephyrs, etc.).

I think the only cards I haven't talked about are Veranus which is here for the board clear combo with Unleash the Hounds, and Maiev Shadowsong which can help you stall until the Turn 8 swing or can help you punch through taunts for lethal once Zixor's been unleashed.  Oh, and Unleash the Beast is here to fill in the 6-drop slot with something that immediately affects the board while also serving as a nice back-up target for Kael'thus on Turn 8.

Substitutions

This deck is far from optimized at the moment and I imagine it'll need some tweaks before I get to the Diamond tier.  The most salient targets are:

Bonechewer Brawler - Originally, I wanted a 2-drop I could play on curve and there was more aggro at the start of this season.  However, at the moment, I think I could cut this down to a 1-of and still be fine.

Marked Shot - I wanted another 4-cost card and liked that this could help with both board control and the consistency of activating Kael'thas combos.  However, it's rarely been helpful for the combo in my limited experience and if this continues I'll probably cut it from the next iteration.

So I think my first replacements will be to sub out 2x Bonechewer and 2x Marked Shot with 1x Doomsayer, 2x Bone Wraith, and 1x Cobalt Spellkin (worse immediate impact compared to Marked Shot, but guarantees a 1-cost spell).  

If you want to be even more competitive you'd probably replace the Kael'thas Sunstrider package altogether (KS, Rapid Fire, Unleash the Beast, and Nagrand Slam). 

In a previous iteration I tried fitting in Tundra Rhino + Scarlet Webweaver for an OTK combo with Zixor Prime, but I didn't like how it muddied the targets for Scavenger's Ingenuity, Scrap Shot, and Ramkahen Wildtamer.  However, if you remove the Kael'thus package you could probably build out in this direction more consistently (by including, e.g., Fresh Scent, Kill Command, and/or Hunting Party).

Please leave a comment below if you have questions about specific substitutions or want to share your own version of this archetype!

Happy Hunting :)