Trisphee - Puzzle & Dragons

That dungeon is ridiculous. Normally when you come across monsters with super high defense your instinct is to bring in as much spike damage as you can so you can overcome it. Extreme King Metal Dragon, however, has DEF that's ridiculous beyond ridiculous -- you have to be able to deal a single hit of over 1.55 MILLION DAMAGE to be able to kill it. Stacking on that kind of damage without a defense breaker is crazy hard. (I crunched the numbers: a fully maxed-out Athena on a 25xATK team using her active skill and King Flamie could do it by matching three sets of four light orbs. This is plausible, but challenging.)

This means that the only plausible ways to do it are abilities that either ignore defense or reduce defense.

Strategies behind the spoiler:

EKMD has too much HP (a little under 45k) to take it out with the direct-damage defense-ignoring abilities available in the game; Darkseid's 8000 damage is the highest there is in the US. (Japan has Bahamut with 9999, but that's still not enough.) So scratch that idea.

25% defense down is really easy to come by -- the Bomb ability available on the demons that drop as trash in pretty much every normal dungeon does 25% defense down. Still need 1.17M damage. 25% isn't enough to avoid needing a 25xATK team, although it does make the necessary combo to deliver the blow easier.

50% defense down is also really easy to come by. The elemental Fighters/Knights/Barons that drop in normal dungeons provide 5 turns of 50% defense down. The fact that it lasts five turns is actually very useful for this dungeon as a whole because it's full of King and Super King dragons, so this is a reasonable option. It brings the damage requirement down to 795k, which is within the realm of possibility for a 9xATK team -- a little challenging, but possible with the right monsters.

75% defense down is pretty rare. As far as I know, only three monster lines have it: Machine Golem, Dark Golem, and Awoken Shiva. This requires a 420k hit, which actually isn't substantially easier than the previous hit because you still need to pack the same set of skills to hit that number, you just can get away with lower base stats.

100% defense down is available on three monster lines: Droidragon, non-Awoken Shiva, and the Holy Dragon Stone from the Shinrabansho collab from a couple weeks ago. This is by far the easiest and safest way to do it, because you don't have to worry about spike damage; you just need to be able to put out 45k damage across your entire team, and that's EASY.

Finally, poison is an option. Your poison has to be strong enough to take out EKMD within six rounds, because he'll one-shot you if it doesn't. This rules out Lilith's poison, but plenty of monsters have 10xATK poison, and Cleopatra has 8xATK.

The problem with the poison and the 75% and 100% defense down skills is that the monsters are only available through the REM or event/coin dungeons, so it's luck-dependent on whether you can actually get the monsters. The benefit of these techniques is that you can stack HP/RCV instead of ATK so it's much, much, MUCH safer to stall to get your skills charged up.