Putting Sylveon in the active spot can put many opponents in a corner if they cannot find a way to damage it, allowing for enough time to give Xerneas the energy for its big Rising Horn attack
If you need another removal option, consider PRB02-001-koby a 4-cost, 5000-power character that brings both tempo and value
Don't copy a rare Pokemon
Im running two in this deck simply because I like other locations that gain lore passively to be included in my strategy, but I could see this being as much as a four-count in your deck if you particularly like this strategy
There's the Rainbow Feather and Silver Feather, the Squirt Bottle, used to wake Sudowoodo, and a range of berries that debuted in the game
There have definitely been games that I could have won earlier if I had a cheap haste enabler online