The “Koa'ki Meiru” archetype revolves around Iron Core of Koa'ki Meiru; the vast majority of the monsters self-destruct during the End Phase unless you send this Spell Card to the Graveyard or reveal a certain type of card from your hand.
Revolution Beginning, the 17th Main BOX, has finally given us, together with some of the monsters we were still missing, one of the best support cards this archetype has ever seen: Diamond Core of Koa'ki Meiru. Koa'ki Meiru decks are now significantly more consistent and, as a direct result, more threatening.
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Use Diamond Core to add Iron Core of Koa'ki Meiru, Koa'ki Meiru Urnight or Koa'ki Meiru Maximus from your Deck to your hand. You can’t add another Diamond Core to your hand, but there is no restriction on how many you can activate in a turn.
If all of this wasn’t enough, you can banish Diamond Core from your Graveyard to prevent all “Koa'ki Meiru” monsters you control from being destroyed this turn. You can activate this effect the same turn you activated Diamond Core and make your monsters immune to cards like Treacherous Trap Hole, Widespread Ruin and Super Rush Headlong.
Since your monsters can’t be destroyed by battle this turn, you can also safely attack monsters with the same ATK with them. Most of the times you do not want to add Koa'ki Meiru Sandman to your hand with Diamond Core because Koa'ki Meiru Urnight can Special Summon it from your Deck.
As I said in the Overview, the “Koa'ki Meiru” archetype revolves around a card in particular: Iron Core of Koa'ki Meiru. Iron Core can be recycled from the Graveyard via its effect, so, even if a monster requires you to discard it, you are not going to run out of them, but the case is different with Koa'ki Meiru Maximus because this Level 8 monster can only be Special Summoned by banishing the Spell Card.
If Iron Core is banished, there aren’t many ways to retrieve it consistently. In addition to requiring you to banish Iron Core in order to be Summoned, Maximus doesn’t just ask you to reveal a card during the End Phase: you will have to discard a copy of Iron Core or a “Koa'ki Meiru” monster, otherwise it will destroy itself during the End Phase.
This deck is built to make explosive, aggressive plays, so it generally doesn’t care about maintaining this monster, but, thanks to Diamond Core, you don’t necessarily have to discard anything during the End Phase.
Once per turn, Koa'ki Meiru Maximus allows you to destroy a card your opponent controls. You can destroy a Set Spell/Trap Card or a monster and, if you have more than one Maximus on the field, you can use this effect with both of them.
Maximus alone isn’t enough to deal with everything your opponent has on the field and, even though it does have a lot of ATK, it’s not going to be enough for an OTK.
Once per turn, Koa'ki Meiru Urnight can Special Summon a Level 4 or lower “Koa'ki Meiru” monster from your Deck, except another Koa'ki Meiru Urnight, if you reveal an Iron Core from your hand. Use Koa'ki Meiru Urnight’s effect to Special Summon Sandman and then Special Summon Koa'ki Meiru Maximus. Now you have a board full of monsters, Maximus’ destruction effect and Sandman’s negation for a Trap Card.
The Balance version of the deck is the most consistent one, but, since it can’t run Sphere Kuriboh or Winged Kuriboh, it’s also the easiest one to OTK on Turn 2. A Turn 1 Koa'ki Meiru Urnight into Koa'ki Meiru Wall to negate Diamond Core of Koa'ki Meiru or Hey, Trunade! and a face-down Paleozoic Canadia or Floodgate Trap Hole is the safest play this deck has, but it’s not always going to be available.
A single Sphere Kuriboh is enough to stop an OTK from the Balance version of the deck, even when the opponent has Koa'ki Meiru Urnight + Iron Core + Koa'ki Meiru Maximus.
If it opens with Winged Kuriboh or Flute of Summoning Kuriboh, the Last Gamble version of Koa'ki Meiru is usually the one with the upper hand: stalling for a few turns means that the opponent will eventually not be able to keep all of their monsters on the field, which will make it easier to OTK them after the 5th Turn.
Buster Blader is one of the worst possible matchups for Koa'ki Meiru: Destruction Swordsman Fusion can get rid of Koa'ki Meiru Maximus and, if DNA Surgery in on the field together with Buster Blader, the Destruction Swordsman, you won’t be able to use Koa'ki Meiru Ice to get rid of the latter. Current versions of Buster Blader run Winged Kuriboh to stall until Last Gamble is available, which is another card that gives Koa'ki Meiru a hard time.
This particular matchup can go both ways: if you can force your opponent to activate their Uending Nightmare, Twister or Double Cyclone with a Hey, Trunade! on Turn 2, then Maximus and Ice can get rid of Ancient Gear Reactor Dragon. Even if Ancient Gear Fortresss is on the field, Maximus can still destroy it and leave the rest to Ice, which can be searched by Diamond Core and Urnight. If your opponent is running Double Cyclone and you are running nothing that they can target during the Battle Phase, they can’t destroy their Geartown/Fortress and Summon Reactor.
Going first, as I said, is pretty rough for this deck: it doesn’t automatically translate into a loss, but it’s certainly not an easy situation.
Cosmic Cyclone and Paleozoic Canadia are very good cards you could add to your deck to either get rid of Vampire Kingdom as soon as it hits the field or to flip Vampire Grace face-down and prevent it from activating its effect;
Treacherous Trap Hole and Sphere Kuriboh are the cards that I have chosen for the Example Deck, but any generic form of backrow is just as viable. If your opponent goes first, they will find themselves in your same situation, open to an OTK and forced to rely on their backrow, but, if they manage to survive, they will be able to use Vampire Vamp to get rid of your main monsters.
Just like with Vampires, this matchup is heavily influenced by the coin toss: both decks focus on finishing off the opponent by making early, aggressive plays, so whoever goes second, will usually end up winning the duel. However, if the Koa'ki Meiru deck is not able to OTK the opponent on Turn 2, due to how fast it tends to run out of resources, it might still lose the duel.
Blue-Eyes decks hardly run any backrow, so you shouldn’t have trouble dealing with them. Maximus has less ATK compared to a Cosmo Brain that was Special Summoned by discarding a Level 8 monster and it ties with Blue-Eyes White Dragon, but its effect and The Tie that Binds! can fix this issue. Keep in mind you can safely attack into a Blue-Eyes White Dragon with Maximus if you have used Diamond Core’s effect from the Graveyard.
Silent Magician can negate Diamond Core of Koa'ki Meiru and, if you destroy it, your opponent will get a 3500 ATK monster on the field that Maximus won’t be able to get over naturally. Spellbook of Fate can get rid of Maximus or Urnight, depending on what your opponent finds more threatening, as soon as it hits the field, so you will have to use Hey, Trunade! before Summoning your monsters, if you have it available. On the bright side, if your opponent does not start with Silent Magician, you can instantly gain control of the field on Turn 2 by simply being able to Summon more monsters than they can banish.