Solgaleo for the BKT-FLI format I'm assuming.
First, I don't think you need a whole playset of Solgaleo-GX's, I'd probably cut just 1 Solgaleo-GX. If you can find one through other methods like Ultra Ball, then you should be fine. Solgaleo-GX isn't the card you want in your hand turn 1, as you'd rather try to set up your bench turn 1 and then find the Solgaleo for later. If you wanted to try it, Alolan Vulpix can be an easy way to find Solgaleo-GX since once you set it up with Brigette, all you have to do is attach a metal energy to the active Pokemon to pay for the retreat cost (In most cases a Cosmog), go into Alolan Vulpix and search whatever you need, not just Solgaleo but cards like Tapu Lele to unbrick your hand when you don't have a draw supporter previously. Speaking of Tapu Lele, if you had the budget I'd probably try to add a third one. Given that this is a pure Solgaleo build, you should have all the bench room to utilize Wonder Tag however you please, which overall just increases consistency for the deck. I might argue to even have a 4th copy, but that means you could be leading with Tapu Lele-GX much more often in the beginning and once you got Solgaleo set up, consistency and set up cards shouldn't be that vital, so I'd just say 1 more Tapu Lele.
I could kind of see the baby version of Dusk Mane Necrozma being in here, but I don't think you need a second copy with Dusk Shot not being that good since you already have the ability to KO almost anything and the second attack's prize trigger probably won't happen to often. If you were at one prize then the last Pokemon you should have out is a GX to get the most out of the "7-prize game" after putting into the active a baby Pokemon like Alolan Vulpix and then having a whole board of GXs or EXs afterwards.
As far as trainers go, for the most part it looks solid, except for Mallow. There's no Zoroark or Octillery in this list to get the immediate benefit of getting the cards from Mallow into your hand the turn you play it down, and even if we're hoping to just simply draw the cards off Mallow the next turn, your deck could be shuffled by an opposing N therefore wasting your supporter for the turn. Parallel City is also a pretty good card, not sure how it would work in this list. This one I could be wrong on but Solgaleo wants targets to blow-up and start taking prizes preferabbly on GX or EX pokemon, and if the opponent gets to ditch something like a Tapu Lele-GX once Parallel City has been activated to limit your opponent's bench, esspecially against Solgaleo, that's less prizes for you. Now you could Guzma whatever's important to the active and then play a Parallel City to limit their bench, as a well timed Parallel can still cripple strategies, but I'm just not too sure how often that would happen. Parallel City is still good, I'd just watch how you'd play with the card in this deck or maybe cut a copy. If you found the room by this point, I'd say add in at least 2 copies of Choice Band since it will help your Prism Star Solgaleo hit important numbers such as 190HP for Buzzwole-GX and Tapu Lele-GX and help Solgaleo-GX knock out literally anything (technically speaking, except for Alolan Golem-GX as he resists metal therefore taking only 240 but no one plays him so you're fine
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After that, nothing really wrong with the energies as 9 basic metal energies is a decent number for Sol Burst and Super Boost Energy is good even if we decided to cut the 4th Solgaleo since it still counts as a metal energy when attached to a Solgaleo. So I guess if you wanted to keep the 4th stage 2 to try and make Super Boost work under something like ability lock that's fine, but if you wanted a 4th stage 2 pokemon for that card, I might try the baby Solgaleo from Guardians Rising, as it's another baby Pokemon to hit something like SHL Hoopa or GRI Alolan Ninetales which would otherwise block whatever you're throwing at them, and once you set up Fangs of the Sunne, you're doing 170 damage, you keep the energy, and you can reset the condition of not attacking next turn with Ultra Road, Guzma, or paying 1 retreat cost from a Cosmog after switching.
All I got to say at the moment. Solgaleo is super underrated in my opinion and I'm surprised he doesn't get utilized more, but then again, the GX is only good for hitting other GX pokemon only as you don't want to use Sunsteel Strike on baby Pokemon. He's usually the kind of card that wants a partner for something like charging him up or giving him extra consistency. Not to say that pure Solgaleo is a bad deck, he's usually just better with any form of partner.
