Stay 'Rost-y (Masters)

Saw someone on ladder playing Rost, the Walking Glacier + Sling of the Chi, realized it was waaaay sweeter than what I was playing, and put this together. Took me from D3 to Masters in a few days, landing in top 100.

Card explanation:

Sling of the Chi - the main build-around of the deck. It provides removal, card draw, and direct damage (with overwhelm units), while also incidentally protecting your stuff from transformation. Needs chonky boiz to enable it, which leads us to...

Beef
Rost, the Walking Glacier - the other build-around of the deck. Works amazingly with Sling, since its massive body lets it both shoot things down and draw cards. If your opponents kill it, it comes back to shoot another thing down, and while it's shifted out, you get to draw even more cards until it can start beating down again. Warp is the icing on the top. It does require an almost mono-primal powerbase to run though.
Siege Breaker - only triggers half of Sling, but killing relics is quite relevant. It's also just a big body.
Geminon, the Double Helix - another massive slab of beef. Surprisingly resistant to removal, since the most commonly-played removal options are damage based or monofaction-only like Send an Agent. FFF can be a bit tricky at times though.
Also tested: Thundrus & Snooze and Daraka, Queensguard, which I was underwhelmed by - they just weren't good enough without Sling active.

Removal
Permafrost - cheap and efficient way to turn off many threats.
Maveloft Huntress - another cheap way to kill things. Plunder is also very valuable to allow you to hit power drops.
Hailstorm - obligatory anti-aggro card. Doesn't kill any of our beefy units, and Jarral Ascending and Maveloft Huntress can both dodge it when pumped.
Kenna, Uncontained - eats slightly larger things and provides an evasive threat. Like Geminon, quite resistant to removal.

Market Stuff
Transpose - extremely well-positioned in the current meta, since Exploit is everywhere. Also protects Sling from removal, and our face from burn in general.
Jennev Merchant - not the most amazing statline, but more market access is always good. Howling Peak Smuggler is another option, but I appreciate the aegis that lets Jennev Merchant serve as a better speedbump against aggro.
Jotun Hurler - market and plunder fodder, but the Snowball is also useful for pinging down aggro units and popping aegis.
Xo of the Endless Hoard - another market / plunder option. Drawing an extra card with Treasure is nice, and if you ever get to eight power (and FFFF influence), Xo is a massive threat that also triggers Sling.

Other Stuff
Dazzle - counters most relevant spells, protects our stuff from removal, plunders. I don't think it's amazingly positioned at the moment, hence the 2-2 split with Permafrost.
Jarral Ascending - a cheap threat and some incidental card advantage. We run enough spells that it can provide some card advantage, especially when you add in Xo and Jotun Hurler, but it's actually primarily here to eat removal so our beefy units are more likely to live. Its other use is to serve as an early blocker against aggro, and to be an imbue target for Maveloft Huntress so she can eat larger things.
Wisdom of the Elders - some actual card advantage for longer games.

Market
Furnace Mage - eats opposing relics, and particularly necessary for the Sling mirror.
Honor of Claws - usually a safe pick, providing some more card advantage and can throw away our various fodder.
Mortar - kills sites, which we can otherwise struggle with.
Poaching Drake - another anti-aggro option, shrinking a opposing threat and providing a blocker. Probably the card I'm most on the fence about.
Turn to Seed - removal for bigger units.

Powerbase: mostly mono-Primal for Rost and Jarral Ascending, but also including enough Fire influence for our splash cards, and Cobalt Waystone as another way to get face-aegis.

Gameplay

This is a midrange deck. Our primary gameplan is to provide some disruption in the early game so we can win with our beefy threats in the lategame. Sling of the Chi provides an incredibly powerful removal and card advantage engine, and we run a few ways to protect it with both face-aegis and Dazzle. If you don't have Sling available (or don't have units to trigger it), it's going to be more difficult to win... but Rost and Kenna are more than capable of dominating a game by themself.

For the mulligan, look for power - two power hands are almost never keepable, but can be keepable if you have Maveloft Huntress or Dazzle to plunder into a third power. One of the most common ways for this deck to lose is to not hit its power drops and have a bunch of beef stranded in hand, but if you can curve Sling into something big, you'll usually be in excellent shape.

Keeping power open for Transpose and Dazzle is often a good idea on turns 1 and 2, since Exploit is pretty common. Don't run out Maveloft Huntress unless there's something to kill or you need to plunder to hit your power drops - it's much more valuable as a lategame piece of removal than as an early beater. It's also much better to plunder away fodder (Jotun Hurler or Xo) when possible.

In the midgame, you usually want to start developing threats like Siege Breaker and Rost. Rost is always excellent to run out due to its ability to recur after being killed. You may also start to have other threats like Jarral or Kenna active - pay attention to sequencing with them (ex: scouting spells to the top for Jarral, playing Kenna before combat so she reaches her ultimate faster).

In the lategame, look to close things out - we don't have the raw card draw of a dedicated control deck, nor do we have lifegain to stabilize against aggro. Face aegis and Dazzle both help buffer against surprises, so try to preserve them.

Closing

After the 3F Cylix and Krul, Xumuc Occultist nonsense that had taken over the ladder for the last month or so, it was really refreshing to get back to my roots - midrange piles of beef with incidental card advantage. Heart of the Vault may be nerfed, but its successors certainly feel up to the task.

Stay frosty, friends!