I've been testing this deck concept quite a bit lately and it's been seeing a ton of success! I do have some changes I've made, as people often do, but the core of the deck idea is very sound
It actually feels like a smooth running meta deck. I hope this type of deck has a strong showing at worlds this year.
Changes I've made:
1. Remove Ultra Necrozma and metal energy, malamar decks are difficult to setup up as it is, adding a second type of energy adds a extra challenge. You don't need Ultra Necrozma as there are plenty of other decent attacks available
2. Double up on Latios-GX it's not only has the most important attack for this deck but it's also a free retreater which is key in malamar decks
3. More Hapu and Acro Bike! As a pokemon veteran once told me, "deck thinning is deck winning"! ...and we need rapid discard options in this deck
4. You only need 2 malamar on the bench, so 3 malamar in the list is fine
5. Reset stamp is a game changer - this card will win you matches in the late game
6. Max count of Cherish Ball - getting that Dedenne-GX or needed tech is key
7. Tech: Dragonite-GX UNM - this card will close out games for you, 270 takes down every Tag Team I can think of
8. Tech: Espurr UNB - this card can finish off a damaged Tag Team on the bench, it's insane, it replaces Mimikyu as a surprise tech
9. Tech: Muk & Alolan Muk-GX - The poison and healing attacks are very strong and can also get around the Latios-GX blocking attack
10. Supporter count 9 or 10. This deck can run dry if supporters can't be chained back to back, a higher count is better
11. Everyone is now running Custom Catchers, they are not a must since you have snipe attacks but they can take surprise knocks outs
12. Add in Jirachi-GX for it's ability, it will help you in the mirror match or against Reshizard with Mew and Mewtwo-GX teched in. It's GX attack is sweet too
Here's the modified list:
##Pokémon - 20
* 1 Naganadel-GX UNM 230
* 1 Dragonite-GX UNM 229
* 1 Dedenne-GX UNB 57
* 1 Espeon & Deoxys-GX UNM 72
* 1 Espurr UNB 79
* 1 Gengar & Mimikyu-GX TEU 164
* 1 Giratina LOT 97
* 4 Inkay FLI 50
* 1 Jirachi-GX UNM 79
* 2 Latios-GX UNM 223
* 2 Mewtwo & Mew-GX UNM 222
* 1 Muk & Alolan Muk-GX UNB 196
* 3 Malamar FLI 51
##Trainer Cards - 31
* 4 Cherish Ball UNM 191
* 4 Mysterious Treasure FLI 113
* 3 Viridian Forest TEU 156
* 3 Hapu UNM 200
* 2 Lillie SUM 147
* 4 Custom Catcher LOT 171
* 4 Acro Bike PRC 122
* 1 Reset Stamp UNM 206
* 4 Cynthia UPR 119
* 2 Switch BCR 135
##Energy - 9
* 9 Psychic Energy XYEnergy 8
Oh and if it's not obvious, Mewtwo & Mew-GX are the main attackers
Unless you are ability locked, then bring in Giratina.
I wanted to mention that I underestimated Espeon & Deoxys-GX. Using it's GX attack with 6 energy has closed out games for me. 200 snipe damage against non-gx decks, like the new Sleep Lock Breloom deck, has come in so handy. Since damage counters are placed it gets around Mew's bench protection ability as well.
As a last note, I just wanted to reiterate the importance of Latios-GX, it's attack paired with Mewtwo & Mew-GX will win you most Tag team deck match-ups.