Magnezone works specifically because of the mechanics of its pre-evolutionary forms. The new variant of Magnemite found in the Space-Time Smackdown expansion only inflicts 10 damage, but you can use any type of Energy to activate it. Truthfully, you’ll hardly ever use this attack, but it’s nice to have the option in case it’s all you have in the early game. The real hero here is the Magneton variant found in the base Genetic Apex set, all because of its ability Volt Charge. This attaches an Electric Energy to the magnet monster every turn (so long as you remember to use it). Magneton and Magnezone both only need one Electric Energy, so you can still use Dark-type Energy to build up their attacks.
In a perfect world, the Darkrai ex and Magneton strategy looks like this: lead with a wall like Druddigon or Kangaskhan, and keep Darkrai ex and Magnemite on your bench. Evolve Magnemite into Magneton, stall the game so Magneton can passively accrue Electric Energy, and keep attaching Dark Energy to Darkrai ex to chip away at your opponent’s health. Switch to Magnezone once you can (either after your wall is knocked out or by using the Leaf Trainer Card), and start laying the smack down with its 110 damage Thunder Blast attack. If Magnezone goes down, or it runs out of Electric Energy, bring in Darkrai ex to clean up the rest of the game. Because Darkrai ex is the only ex card in this deck, it’s best used after your opponent has already scored two other KOs, unless you know you can lock in a victory by bringing it in later.
What makes Magnezone so deadly is that, even if you can’t stall, it’s still very powerful. For example, let’s say you start the game with a Magnemite, Magneton, and Magnezone. Evolve Magnemite into Magneton as soon as you can, and activate Volt Charge upon evolution. Then on your following turn, make sure to use Volt Charge again before evolving Magneton into Magnezone, and just like that, you have two Electric Energy that are good for two uses of Thunder Blast. Slap Dark Energy on it to meet the three Energy minimum requirement, and you can immediately begin to pick up easy KOs. Alternatively, if your opponent has a relatively weak monster on the front lines, you can keep Magneton in play and use Spinning Attack to pick up a KO without consuming valuable Electric Energy. It’s surprisingly versatile!
Getting doubles of each Pokémon to use in a deck is undoubtedly a pain (I’ve been working with a single Darkrai ex myself, and I consider even that lucky). But once you’ve assembled those pieces, all you need are a handful of trainer cards to complete your deck. Professor’s Research, Poké Ball, and Leaf are common staples in Pokémon TCG Pocket, and you’ll want two of each of them. You only need one Cyrus, and a Giovanni can help you lock in KOs. From there you can slot in a Sabrina, Dawn, Giant Cape, or Rocky Helmet to round out your support. Just keep in mind that, as the competitive landscape evolves, other Trainer Cards may become more prevalent to counter new threats.