Trying out Sylveon instead of Diancie and/or Alolan Vulpix. I think Diancie, and to a lesser extent, Alolan Vulpix, is more consistent in getting Gardevoir out quickly, but I do recognize that Sylveon has its merits. Plea GX really helps against Metagross and Gardevoir mirror matches, especially those that don't run Sylveon themselves. Magical Ribbon is also the strongest non-GX search attack in the game. There are a couple "flex" spots in this deck, which are the Energy Retrieval, the third Eevee, and maybe the Skyla. Let me know if you'd make any changes.
****** Pokémon Trading Card Game Deck List ******
##Pokémon - 19
* 3 Eevee SUM 101
* 1 Oranguru PR-SM SM13
* 2 Sylveon-GX GRI 92
* 4 Ralts BUS 91
* 2 Kirlia BUS 92
* 3 Gardevoir-GX BUS 140
* 1 Gallade BKT 84
* 3 Tapu Lele-GX GRI 137
##Trainer Cards - 29
* 1 Rescue Stretcher GRI 130
* 3 Choice Band GRI 121
* 3 Professor Sycamore BKP 107
* 1 Professor's Letter BKT 146
* 3 Guzma BUS 115
* 4 Ultra Ball SUM 135
* 2 Float Stone BKT 137
* 1 Energy Retrieval SUM 116
* 1 Brigette BKT 161
* 1 Skyla BKT 148
* 2 Field Blower GRI 125
* 4 N NVI 92
* 3 Rare Candy SUM 129
##Energy - 12
* 4 Double Colorless Energy SUM 136
* 8 Fairy Energy 9
Total Cards - 60
****** Deck List Generated by the Pokémon TCG Online www.pokemon.com/TCGO ******
****** Pokémon Trading Card Game Deck List ******
##Pokémon - 19
* 3 Eevee SUM 101
* 1 Oranguru PR-SM SM13
* 2 Sylveon-GX GRI 92
* 4 Ralts BUS 91
* 2 Kirlia BUS 92
* 3 Gardevoir-GX BUS 140
* 1 Gallade BKT 84
* 3 Tapu Lele-GX GRI 137
##Trainer Cards - 29
* 1 Rescue Stretcher GRI 130
* 3 Choice Band GRI 121
* 3 Professor Sycamore BKP 107
* 1 Professor's Letter BKT 146
* 3 Guzma BUS 115
* 4 Ultra Ball SUM 135
* 2 Float Stone BKT 137
* 1 Energy Retrieval SUM 116
* 1 Brigette BKT 161
* 1 Skyla BKT 148
* 2 Field Blower GRI 125
* 4 N NVI 92
* 3 Rare Candy SUM 129
##Energy - 12
* 4 Double Colorless Energy SUM 136
* 8 Fairy Energy 9
Total Cards - 60
****** Deck List Generated by the Pokémon TCG Online www.pokemon.com/TCGO ******