Darkrai EX
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Darkrai EX is a DECEPTIVELY strong card. While 80 damage at 3 energy is nothing particularly special, the fact that it automatically deals damage makes it effectively up to 100 damage while it's on the front line, and also gives it utility of dealing damage from the bench. Darkrai EX single-handedly put Darkness decks on the map, and it's undeniably great!
Darkrai EX and MagnezoneGo to Deck
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Darkrai EX and Magnezone have stupid-good synergy, though the real highlight card here MAY be Magneton. While Darkrai sets itself up and takes pot-shots, Magneton will generate its own Electric energy, meaning you'll likely have 2 high-performance cards set up and ready to go at about the same time at the cost of 1. Druddigon can act as a powerful wall that racks up bits of damage while everything gets set up.
Note: you should only have Dark energy on this deck, as Magneton is self-sufficient in terms of Electric energy.
Darkrai EX and Weavile EXGo to Deck
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Darkrai EX and Weavile EX have a lot of synergy. The former can sit on the bench and set up while dealing damage, making way for Weavile EX to come in early and start mowing the lawn. Rocky Helmet also helps with chip damage. Spiritomb may seem like a slightly odd pick here, but it serves two purposes: first is as an early-game wall with a low retreat cost. If you put a Giant Cape on it, then it reaches the coveted 100 HP benchmark, allowing it to survive at least one attack before dropping against the majority of decks. it also hits the entire bench for 10 damage, which opens the field for Weavile EX to hit for maximum damage, and for Cyrus to pull in any card at will.