Expanded Zen Arrow (Decidueye Alakazam)

[Alolan Vivillon]

Pokémon - 15

* 3 Rowlet SUM 9
* 1 Dartrix SUM 10
* 3 Decidueye-GX SUM 12
* 1 Magearna UPR 91
* 3 Alakazam-EX FCO 25
* 1 Tapu Lele PR-SM SM45
* 3 M Alakazam-EX FCO 26

Trainer Cards - 38

* 3 Dimension Valley PHF 93
* 2 Professor's Letter BKT 146
* 2 Alakazam Spirit Link FCO 90
* 3 Choice Band GRI 121
* 2 Field Blower GRI 125
* 2 Energy Retrieval SLG 59
* 2 Ultra Ball DEX 102
* 3 Devolution Spray EVO 76
* 3 Rare Candy SUM 129
* 2 Acerola BUS 112
* 2 N FCO 105
* 2 Cynthia UPR 119
* 2 Professor Sycamore BKP 107
* 2 Guzma BUS 115
* 2 Pokémon Fan Club UPR 133
* 2 VS Seeker PHF 109
* 2 Pokémon Communication TK-Excadrill 24

Energy - 7

* 7 Psychic Energy XYEnergy 8

The primary strategy of this deck is to scale damage up with Decidueye, then use Zen Force to hit insane amounts of damage. Magearna is used to pull the Alakazam Spirit links off, then once the new Tapu Lele is released as a Psychic type, it can reap the benefits of Dimension Valley.
All I need is a single Psychic Energy and Dimension valley in play, then from there I'm able to attack for one energy. Once they have four Damage counters on them, with choice band I'm hitting for 160, plus the four on them is 200. It can easily one hit Buzz and Naganadel. Gardevoir has a harder time getting OHKO's because of the single energy cost. My primary concern is getting out the Decidueyes now that FTGP is banned. Any suggestions?

 

[Christopher Shehan]

I think this is a pretty cool idea. I would run mega turbo instead of the energy retrevial. I would up the spirit link count just because you always want to have it when you evolve. Nest ball is probably better than OFC. That way you can use a draw supporter for the turn. Hoops ex is pretty good as is Shaymin ex but it starts to clutter your bench unless you want to run a couple sky fields

 

[Alolan Vivillon]

I think this is a pretty cool idea. I would run mega turbo instead of the energy retrevial. I would up the spirit link count just because you always want to have it when you evolve. Nest ball is probably better than OFC. That way you can use a draw supporter for the turn. Hoops ex is pretty good as is Shaymin ex but it starts to clutter your bench unless you want to run a couple sky fields

I completely forgot of the existence of Mega Turbo.
I agree with the higher amounts of Spirit link, and I may test nest balls.
Hoopa seeks like a good idea, thanks for that.
Not going to do shaymin cause I don't want to risk that. And if push comes to shove, Hoopa could have an insane attack to work with Alakazam

 

[VioletValkyrie]

Pokémon - 15

* 3 Rowlet SUM 9
* 1 Dartrix SUM 10
* 3 Decidueye-GX SUM 12
* 1 Magearna UPR 91
* 3 Alakazam-EX FCO 25
* 1 Tapu Lele PR-SM SM45
* 3 M Alakazam-EX FCO 26

Trainer Cards - 38

* 3 Dimension Valley PHF 93
* 2 Professor's Letter BKT 146
* 2 Alakazam Spirit Link FCO 90
* 3 Choice Band GRI 121
* 2 Field Blower GRI 125
* 2 Energy Retrieval SLG 59
* 2 Ultra Ball DEX 102
* 3 Devolution Spray EVO 76
* 3 Rare Candy SUM 129
* 2 Acerola BUS 112
* 2 N FCO 105
* 2 Cynthia UPR 119
* 2 Professor Sycamore BKP 107
* 2 Guzma BUS 115
* 2 Pokémon Fan Club UPR 133
* 2 VS Seeker PHF 109
* 2 Pokémon Communication TK-Excadrill 24

Energy - 7

* 7 Psychic Energy XYEnergy 8

The primary strategy of this deck is to scale damage up with Decidueye, then use Zen Force to hit insane amounts of damage. Magearna is used to pull the Alakazam Spirit links off, then once the new Tapu Lele is released as a Psychic type, it can reap the benefits of Dimension Valley.
All I need is a single Psychic Energy and Dimension valley in play, then from there I'm able to attack for one energy. Once they have four Damage counters on them, with choice band I'm hitting for 160, plus the four on them is 200. It can easily one hit Buzz and Naganadel. Gardevoir has a harder time getting OHKO's because of the single energy cost. My primary concern is getting out the Decidueyes now that FTGP is banned. Any suggestions?

I'm certainly a fan of the idea! I don't necessarily consider myself the most proficient in these kinds of lists, but here are the thoughts that came to mind.
First, for your Pokémon:
➤ First of all, I'm very confused about the addition of Magearna UPR, as I understand that you can swap Pokémon tools (specifically Spirit Links) off of Alakazam-EX, but I really don't think it's worth the bench space when I'm assuming you want a set-up of (or similar to) 3 Decidueye-GX, 2 M Alakazam-EX, and 1 Tapu Lele out on the bench. By playing Magearna you make it a lot harder to play your bench space while reaping the effects of a card that you can afford to have accomplished through the means of something like just using Field Blower on your own tools. In theory, you want to have a strong set-up so you never have to rely on using Choice Band to have to reach for that extra damage.
➤ I think a copy of Tapu Lele-GX is worth running, or at least considering for this deck. Because of how Expanded decks typically lay out their Supporter line-ups, you benefit from having that quick search from Wonder Tag in basically every deck in the format.

Now, for trainers:
➤ I personally don't see a reason for you to be playing Professor's Letter, much less two copies of it. I don't think it's necessary.
➤ 2 Alakazam Spirit Link seems like way too little of copies of such an important card to be playing in your deck. I'd raise it to three, or even four.
➤ I believe you're playing with way too large of an emphasis on Choice Band. I would cut it from three, to one at most. I'm going to once again say that in theory, you always want a strong set-up of damage on your opponent's board to ensure you can hit knockouts without the need for it.
➤ For the same reason as Professor's Letter, drop Energy Retrieval. I'm not keen on the idea of playing Mega Turbo as it's very situational, needing a Mega in play as well as the energy in the discard, and you're already attacking for a single energy on M Alakazam-EX. You don't need that energy acceleration.
➤ Either raise your count of Ultra Ball to four, or consider playing Mysterious Treasure [SM: Forbidden Light] to allow you to search out Pokémon easier.
➤ Play Super Scoop Up over Devolution Spray. Sure, it's based on coin flips, but it allows you a lot more flexibility, as well as immediately acting as a full heal to whatever you bring back up, as well as freeing up bench space and getting an unwanted Pokémon out of the active spot.
➤ Drop one Cynthia for a third N. Cynthia is a lot weaker in Expanded due to the sheer amount of options you have for draw support. I'm saying to replace with N specifically to keep your "Shuffledraw ~6" count consistent.
➤ Lower your Professor Sycamore count by one. Expanded also has the resources to run one-count Supporters while keeping consistency due to a wonderful card by the name of VS Seeker.
➤ Speaking of which, definitely raise your VS Seeker count to 4. 2 VS Seeker is way too low, and causes you to have to run extra counts of the supporters themselves, rather than the more flexible and much less situational VS Seeker.
➤ Remove Pokémon Communication altogether. It can be a possibly dead card in your hand if you don't have a Pokémon to put back into your deck, and is easily outclassed by cards like Ultra Ball or Mysterious Treasure.
➤ Add more Supporters. I'd advise adding a Colress, as it's the most adept shuffle draw in the current format.

Besides that, your Energy count is fine.

I do want to point out one thing I find very intriguing reading your OP. It seems like you're building this deck and considering its matchups in the format as if it's a Standard deck, but you seem to be attempting to add cards that are from the Expanded format, and not making full use of them. (As an example, playing only 2 VS Seeker.) You still have a short while to play this deck in the Standard format if you so wish, but I think if you're going to be playing this deck in Expanded, consider your deckbuilding and your deck's matchups across the meta as an Expanded deck in a field of other Expanded decks.

I hope this helps! @Alolan Vivillon