If I may: very nice template, any suggestion from now on would be personal preference or meta-calling.
I'll go backwards and start with energy. If you're shooting for Necrozma, there's barely any reason NOT to include Psychic energy and have it as a secondary attacker, especially if Volcanion is present at your locals.
As far as quantity, you always want 4 Psychic energy, no matter the number of Metal left. I say this because Metagross decks float around 9~11 energy (someone here will probably recommend you to drop 1 or 2 energy cards eventually).
As far as items go, you don't need Switch. You want manual retreating as often as possible and Guzma is enough artificial switching for this deck.
Also, 4 Rare Candy is overkill and increases your chances of dead-drawing into it late game.
For supporters, from experience, 4 Guzma has a tendency of bricking hard sometimes, as important as it is.
Some people will suggest only 1 Brigette, as you only need it once every game, due to precious deck space.
There's also the posibility of reducing Sycamore due to you not wanting to lose some important resources on a bad draw with a Sycamore outlet.
Finally, for pokémon, two Necrozma is unusual, but almost ensures you always have Black Ray GX from the get-go.
Also, there will be some input on 2 A. Vulpix, same reasoning as Brigette.
With those notes in mind, my recommendation would be as it follows:
-1 A. Vulpix
-1 Necrozma
-1 Professor Sycamore
-1 Guzma
-1 Rare Candy
-1 Switch
-5 Metal energy
-------------------
+1 Dhelmise
+1 Professor Kukui
+2 Parallel City
+1 Enhanced Hammer
+1 Max Potion
+1 Super Rod
+4 Psychic energy
One of Metagross' most troubling issues is damage. Nowadays, you want to hit 190 or 210 for the OHKO on key meta elements (most prominently, Turtonator-GX, Buzzwole-GX, Golisopod-GX and Zoroark-GX, the latter duo which happens to be one half of the S-tier tournament field), unless you Black Ray GX them and they don't heal/scoop. With Choice Band, Dhelmise and Kukui, you can hit that. It's a lenghty combination, but reasonably doable as a one-of and potentially game-winning, especially if you are forced into Algorhythm GX and it sticks.
Parallel City is a great option against today's meta. It cripples Zoroark-GX's and Xerneas BREAK's damage cap, if you point the bench-reducing side to your opponent. But it also removes sitting Tapu Lele-GX and A. Vulpix from your side, with a curious side effect: reducing damage from Golisopod-GX and Tapu Bulu-GX, preventing key K.O.'s and screwing their math.
Enhanced Hammer is clutch in such special-energy-oriented meta. 3 Max Potion is an ideal number for extra longevity and brick prevention. Super Rod acts as a second Stretcher, while preventing you from running out of energy late game, where you might not have 3 Metagross on the field and need manual attachments to keep the cycle going.
Some other card choices to consider, if you find room after testing, would be:
Mimikyu (surprise one-prize attacker)
Cobalion (if A. Ninetales / Hoopa are a thing)
Magearna EX (against Espeon EX devolution strats)
Altar of the Sunne (2 copies, at least, against Volcanion EX)
Field Blower (a 4th copy if Garbotoxin runs rampant)
Skyla (to search for a key Rare Candy or Choice Band)
As for running a Metagross-GX deck itself, you're trying to set 3 of them up as fast as you can, always keeping in mind that you're playing a slow game. Once you get them online, try to always have at least one Metagross-GX with 1 energy on the bench. If one of them goes down, you can still pump out attacks with 2 running. Also, if you chose to include Magearna EX, it protects them from Espeon EX.
T1 Brigette, as usual, is imperative for the kick-off. If you only have your starter and Brigette pull, your table should consist of A. Vulpix, 2-3 Beldum and either Necrozma or Dhelmise (if 2 Beldum). Once you know what your opponent is playing, you should be sensitive of pulling Necrozma/Dhelmise or not.
If you have 2 Metagross-GX set up by T3, you're in a very comfortable position. But, if the match-up is favorable (i.e. VikaBulu), focus on getting one out and swinging as fast as you can (ideally, T2 with candy) to gain tempo, which this deck needs a lot of.
If Necrozma isn't needed in the matchup, be careful when to drop it because 180hp is almost a free prize if it sits on the bench for too long and you already have Tapu Lele-GX as an easy target (Parallel City pays itself off here). Also, in this case, don't be afraid to Algorhythm GX T2 if things get sour. It's better to force an N from your opponent than to save for Black Ray GX without getting the deck going (and, sometimes, you might be positively surprised by your opponent not having N).
Other than that, it's pretty straightforward. Get 3 out, cycle energies from retreating, smash them hard. Hope this wall of text helps...=D
***EDIT: Just realized I left two cards out, filled with suggested option***
