RE: Fire/Dark Deck
Keep in mind that this list is not a Budget Deck, so be careful if trying to use this kind of deck, since it could cost a LOT of money to make.
-3 Yveltal-EX (XY)
-2 Yveltal (XY)
-1 Darkrai-EX (LTR)
-1 Jirachi-EX (PLB)
-3~3 Raichu (XY)
-2~2 Flareon (PLF)
-1 Leafeon (PLF)
-4 Professor Sycamore
-4 N
-2 Colress
-1 Skyla
-1 Lysandre
-1 Computer Search ACESPEC
-4 Ultra Ball
-1 Battle Compressor
-2 VS Seeker
-3 Muscle Band
-1 Professor's Letter
-1 Escape Rope
-2 Float Stone
-2 Bicycle
-2 Shadow Circle
-8 Dark Energy
-4 Double Colorless Energy
The strategy of this deck is to use Yveltal-EX from XY as a primary attacker. He's very strong, so you'll probably use him most of the time. Darkrai-EX is used to help give your guys Free Retreat, and Jirachi can help scope out a Supporter that you might need. Basic Yveltal is a Secondary Attacker, and can be used to charge up Pokemon on your Bench. Flareon is a Fire Type attacker that will be specifically used to to cover Weakness against Dialga-EX decks and Virizion-EX/Genesect-EX decks. Raichu covers Weakness against opposing Yveltal-EX, Lugia-EX, and is an overall nice attacker for Colorless Energy. Leafeon is a nice additional Tech attacker against things like Seismitoad-EX and Keldeo-EX.
As for the Trainers, it's typical fare you might expect. Though, there are a few cards in particular to mention. Battle Compressor is used to get Pokemon in the discard pile to power up Flareon, or to eventually grab out with Basic Yveltal. Since this deck suffers a lot when your opponent can take advantage of the Lightning Weakness or Yveltal and the Fighting Weakness on Darkrai and Raichu, Shadow Circle is the Stadium card of choice here.
Like I said, this deck will probably be pretty costly (It might even run you up to $100 or more, depending on what cards you already have). There really aren't any other good ideas for Dark/Fire decks out there that are more inexpensive, but you can check out
this thread to look specifically for Budget Deck ideas, and
this thread for other Deck ideas in general.