Rayquaza EX's (EX) Dragon Burst is an incredibly efficient attack that easily Knocks Out anything in the format, but you'll have to be careful to weave Delphox attacks in so that the opponent cannot win by Knocking Out 3 Rayquaza EX for 6 Prize Cards
They can be particularly valuable in white decks, colorless/artifact strategies, or Eldrazi tribal decks where your traditional draw sources might be limited
Black has powerful planeswalkers that generate a resource advantage and the ability to trade cards for life, which are traits that midrange decks appreciate very much
Yidris, Maelstrom Wielder may have a lockdown on four-color cascade on your turn, but Abaddon can grant his chaotic boon to spells cast not even on your turn
Mareanie can be obtained from Mega Rising: Mega Altaria, Mega Rising: Mega Blaziken, and Mega Rising: Mega Gyarados packs in the Mega Rising set
(The pull rates for one of the Mega Evolution golds were one in every 2,250 packs, ffs.) Oh, and especially dont buy from people claiming to sell you a card thats not even printed yet.