11th at Malaysia Premier Ball League
Cynthia's Garchomp ex functions a lot like the Garchomp deck from Black & White Block. You build your board with Gabite's Ability, which gets Garchomps to attack with, as well as damage buffs for them (Altaria back then, Roserade now). Instead of being a 1-Prizer, the Garchomp in this format boasts an amazing HP stat, hitting 400 with Cynthia's Power Weight attached.
Unfortunately, Cynthia's Garchomp ex is unfavored against most of the top decks. Gholdengo ex and Raging Bolt ex can hit the 400 damage threshold for one-shotting Garchomp with enough Energy in their hand/on the board. Dragapult ex can get there with Dusknoir and Jamming Tower support. Gardevoir ex resists Fighting, and Cynthia's Spiritomb infuriatingly ignores Weakness.
Get a bunch of Cynthia's Gibles out and evolve one of them into Cynthia's Gabite. This deck has a very unique setup pattern; once you get a single Gabite down, you can use it to search for another Gabite to evolve one of your Gibles, and the new Gabite can search for another Gabite, and so on. A single Gabite can swarm the field with more Gabites as long as you have Gibles to evolve, ending the chain of searches with another Cynthia's Pokémon of your choice.
When you have to get aggressive, you'll want to look for ways to get ahead in the Prize race avoiding using Draconic Buster as much as you can. That attack discards all Energy attached to Cynthia's Garchomp ex, so you have to be discerning in when you fire it off. You can Boss up evolving Basics/Stage 1s from the Bench and pick them off with Corkscrew Dive (or a Roserade-boosted 70 damage Dragonslice from Gabite, if you don't want to leave the 2-Prize Garchomp in the Active). A Roserade-boosted Corkscrew Dive can 1HKO the commonly played Fezandipiti ex due to Weakness. If Garchomp has taken a hit but survived, you can switch to Cynthia's Spiritomb and reflect the damage back at them.
Cynthia's Garchomp ex is your main attacker. It has good HP (tied with Charizard ex, even without Cynthia's Power Weight), 0 Retreat Cost (letting you retreat to Cynthia's Spiritomb for the follow-up after losing all your Energy), and a good setup/post-Iono attack in Corkscrew Dive. Its tankiness is its best trait; with Cynthia's Power Weight, it can survive even Iron Leaves ex hitting for 360 after Weakness. As long as Garchomp survives a hit, you can exploit the damage on it with Munkidori or Cynthia's Spiritomb. Garchomp's main issue is that Draconic Buster is very hard to use consecutively since it makes you discard all your Energy, and the damage is quite low at 260. That second part can be fixed somewhat with Cynthia's Roserade and the aforementioned Munkidori.
Cynthia's Spiritomb has the potential to hit high damage numbers for 1 Energy. It's kind of like N's Reshiram's Powerful Flame with different scaling (equal to damage taken instead of double); you just need Cynthia's Garchomp ex to survive one big hit, which should be easier since it caps out at 400 HP compared to N's Zoroark ex's 280.
This deck wants to go second, as it's an Arven → Buddy-Buddy Poffin + Technical Machine: Evolution or Nest Ball + Technical Machine: Turbo Energize deck.
Secret Box. It adds setup consistency, being another way besides Arven of finding the Buddy-Buddy Poffin + Technical Machine: Evolution combo. In this deck it's also usable late-game; other decks may struggle to find 3 other cards to discard to it, but Cynthia's Gabite can search for fodder to feed Secret Box's discard cost.
Neo Upper Energy and Unfair Stamp are alternatives, but require different deckbuilding concessions (seen next).
The first question that distinguishes the various Cynthia's Garchomp ex builds is "what Energy acceleration are you running?" Draconic Buster makes you discard all your Energy, so this is a question worth considering. In decreasing order of popularity, the answers are:
Play 2 Crispin. This pushes you towards more Munkidori and basic Darkness Energy. A typical count is 2 Munkidori, 4 Fighting Energy, 2 Darkness Energy, 3 Luminous Energy. Note that Crispin cannot power up Draconic Buster from nothing all by itself, since it searches for different types of Energy. You'll need to have naturally drawn into another Fighting or Luminous Energy (or Earthen Vessel for the former) to manually attach.
Play Neo Upper Energy and 2 Hilda to search for it. Unlike Crispin, which you can potentially use to accelerate Energy twice if you play 2 copies, you can only attach Neo Upper Energy once, so Neo Upper Energy builds will have Switch and Air Balloon to cheat on their other Energy expenditures. A typical count is 1 Switch, 1 Air Balloon, 7 Fighting Energy, 2 Luminous Energy, 1 Neo Upper Energy.
Don't play any acceleration and play Unfair Stamp instead. Try to "gain" a turn to attach Energy by Stamping them and hoping they brick. You may play Powerglass as token acceleration, but note that attaching Powerglass means forgoing Cynthia's Power Weight.
The next question to ask is "what's your Gholdengo ex counter?"
Budew, to keep them off Superior Energy Retrieval. This is the softest counter, since they can use Levincia or Ciphermaniac's Codebreaking to find Energy to KO Budew with Make it Rain, but it can disrupt other non-Gholdengo decks, and takes up the least deck space.
Cornerstone Mask Ogerpon ex, the wall. This fits Crispin builds, since Crispin will be useful in accelerating to 3 Energy for Demolish. You'll want to include 1 Technical Machine: Turbo Energize for the same reason.
1-1 Toedscruel, to keep them off Superior Energy Retrieval. This fits Neo Upper Energy builds, since they have Switch and Air Balloon to bail Toedscruel out if it gets Bossed up (it has a Retreat Cost of 2!).
Team Rocket's Petrel: Another way to search for Secret Box.
Pokégear 3.0: Digs for Supporters. Cornerstone Mask Ogerpon ex prefers this, as it can dig for Crispin to accelerate Energy or Boss's Orders to hunt down the opponent's outs to Ogerpon, depending on the situation.
Ultra Ball, Rare Candy: Arven toolbox targets.
Like Drakloak and Ethan's Quilava, Cynthia's Gabite is a card advantage engine, so you'd want to avoid evolving it into Cynthia's Garchomp ex unless you need to attack with it in the same turn. There's no reason to evolve it out of fear of hand disruption, as even if you're holding Garchomp and it gets Iono-ed into your deck, you can use Gabite to search it out again.
If you are planning to attack with Corkscrew Dive, you'd usually want to hold off on Champion's Call if you're not immediately playing or discarding the Pokémon you search for, since holding that extra card in hand means drawing 1 less card off Corkscrew Dive.
Cynthia's Power Weight can help your Pokémon survive Bench snipes by raising their maximum HP above the snipe attack's damage.
Teal Mask Ogerpon ex hits Cynthia's Garchomp ex for Weakness, but it'll come up short of 1HKOing if you have Cynthia's Power Weight attached, or if you've discarded all your Energy to Draconic Buster.