Underdog game mode is here again (not for long, unfortunately), so I'll share my deck that hasn't let me down in this mode. For context, I'm usually in Mythic, but some games might've been played in rank 10/11. In game modes, you're more likely to meet with players from lower ranks too, so there is also that.
As you can see, it has a whopping 89% winrate over 47 games. I believe mutations are the way to go for this format, as you still have the main generators (Hut & Druid) and Arachne plus Leyline Feeder can still generate the remaining Mutations if needed. The main idea is to not lose board entirely until you can mutate into the really good 8/9 mana spells, or if in trouble, use the 5/6/7 mana spells you mutated into effectively. To master this deck, in addition to the tcg basics of tempo and value, you need to be aware of most Nature spells and their mana cost, and nature's synergies/combos.
Let's start with a quick overview of the deck's content:
2x Wayward Hut - Main mutation generator. It's adviced to not play it until it can generate at least 2 mutations, either with Best Friends/Dogs Unchained/Wilds from hand & GP
2x Best Friends - Best to be used as mutation generator with Hut
2x Spore-addled Arachne - 3/2, 1 mutation, mulligan keep
2x Magitech Aquarium - This kind of carries the deck, as noone runs relic hate in this mode. Usually it's worth to wait till Gorilla at 8 mana, but sometimes if you're really behind, you'll have to use it earlier.
2x Infested Satyr - Always keep it in mulligan, best start of the game, even in turn 1 with a pip
2x Druid of the Vines - If it survives a 2 turns, you're in a good place
2x Dogs Unchained Not only for the cover and title, it's a decent combo piece with hut, the 3 mutations it can generate are great, or good to combo with a low mana Hollow sometimes for 3x 5/5 dogos.
1x Undersea Harvest - Value generator, and mutations actually work on the base spell cost of these spells, so that's also good
1x Zaheer, Endless Bloom Another keep in mulligan, it's really important for having board presence
2x Leyline Feeder Best to use it on Zaheer/Twisted gate or a Pack Succession damaged creature / damaged Rhinos from Path of Crumbs
2x Inkcloud Giant Another mulligan keep most of the time, it's usually a 2 for 1 card
2x Twisted Gate Can stall for 1-2 turns, or opponent might even trade into it. Best to use if you also have Leyline in hand.
2x Sartonian Spearfisher With the generated spell there are 2 things to consider: can you use it now to get a big advantage? If not, just for anything from 5 mana
2x Pack Succession Use it if you're really behind on board and HP, but otherwise keep it for mutations
2x Falling Star Same as Pack Succession, or it's also 3 mutations with Hut on board
2x Enchanted Vines Only use it if it kills valuable enemy board, otherwise wait for mutations
Mulligan
Always keep: Satyr, Zaheer, Arachne
Nearly always keep, (especially): Hut (if you have Best Friends/Dogs Unchained), Inkcloud (you don't have the other keeper cards), Aquarium (if you have at least 1 mutation generator in hand)
Keep most of the time: Druid, Best Friends, Spearfisher (if the other 2 cards are 2 mana creatures)
As you can see going for board early is important, especially since there aren't many clears in this game mode. If going first, I usually pip out Satyr/Arache on the first turn. Especially Satyr, it just provides an amazing tempo most of the time, and it's hard to be punished.
Nature spells you should know
Hollow Form and Path of Crumbs are your main targets for mutations, those are 8 and 9 mana spells, so you'll need at least 2-3 mutations to reach them most of the time. With Hollow, it's best if you have at least 3 target creatures for it in board/hand, otherwise it's always worth to use 1 more mutation for Crumbs. You'll always summon 2 Rhinos with Crumbs, and usually deal 5 damage to 2 enemy creatures. Important to note, it has to be 2 different creatures, and it's a target spell, so can't use it on hidden creatures. The third option is either drawing or heal, depending on your situation.
Other notable spells you usually mutate into:
Photogenesis: Really good if you don't have cards, can make for good Hollow Form targets
Swallow whole: can be good sometimes if opponent has 1 single threat, or if you also have photogenesis in hand, and you can manasurge it with that
Seasonal rebirth: only use it when in serious trouble to avoid lethal damage
Winter's Bounty: If it's against a really aggressive deck, and you're really low on HP can be useful
Opalised Roots: Don't think about keeping it, you'll likely never have the hand space. In the 1 exception you have space and hollow form, you can use it
Hunt/Enchanted Vines: If behind, those can help to clear enemy board.
Selena's Insight: A tricky spell, you need a strong own creature, or really weak enemy board for it to be useful. You need to remember, it has to survive and kill the enemy creature at the same time
Drag down: Can be really good in this format, especially against light late game too
Summary
Obviously, there are a lot of nuances for this deck to work well (you should check all possible nature spells and nature creatures you can summon from Aquarium), and I hope underdog won't become a total mutation fest, as the beauty of the mode is that you see a lot more interesting decks/cards than in Ranked. But let me know if you had any luck with the deck, or have further questions on what to use and when with this deck. Also, let me know if there are more elements of decks/modes you'd like to read about.
Also, here is the deck code for it: GU_1_5_NAKNAKQARQARNAMNAMNCSNCSHAzHAzMBDMBHMAyMAyEAPNADKBQKBQGAmGAmIDOIDOMBMMBMQAmQAmMBDEAPNCUNCU
Good luck and see you at the arena!