Lost Kingdoms/YMMV

Demonic Spiders:
- The Vodianoi. They're fast, they hit hard and have high health and defenses. Fortunately, any earth damage kills them.
- Chariobots. Their movement hurts, and they're resilient as all hell get out. As icing on the cake, you can miss out on getting one if you're not so lucky with the mission results screen.
Ear Worm: The first game's soundtrack is riddled with these. Shame there's no OST for sale.
Game Breaker: Way too many.
- The God himself is a huge game breaker. It brings everything on the field (including yourself and your own monsters) to 1 HP. Can make any boss easy to Cherry Tap.
  - Can make you easy to cherry tap too if you aren't careful. Don't use it against the Demon Fox.
- The Doppleganger. Moves slowly and randomly around the field, but has massive HP and will kill the first thing (even you!) that it touches. Throw it right on top of a slow but hard to take down boss and it takes it down like nothing. Of course, some enemies were given an immunity to it's effects in 2.
- The White Tiger, unlike the other Flash cards, deals massive damage to every enemy on the field. The only way it could be more broken is if it was a Neutral type.
- A lesser example, the Steel Skeleton. Great HP, huge Defense, powerful attacks, and each attack has a chance to petrify or OHKO almost any monster (certain ones are immune, but they are far fewer than you'd think). It's fun to one-shot the Sand Worm 4 times in a row in Gromtull Desert. Plus, doing damage sometimes HEALS it. With some luck, it's entirely possible to beat some of the early-mid levels(basically, anything before Ruh-Arok Temple) with just 2-3 Steel Skeletons and nothing else. Oh, and did I mention that you can get the card that transforms into this after the first level? It takes 5,000 EXP to transform it, but that's nothing that some grinding can't solve.
- The first game pretty much became broken when you disregarded the large deck size limit and instead made a small deck composed of nothing but the most powerful attack cards in the game and "recover used cards" cards.
  - Of note, the Blue Dragon. Restores you to 100% health and shuffles everything back into your deck except for the copy you played. Two of them=Immortality.
- There's also the casting from HP system in the 1st game. You could cast any spell in your deck for free if you were at 1HP.
- The Wizard card doubles your damage, essentially giving you a free Z-Charge. Combined with an appropriate Lizardman card and most enemies can become 2-3 hit kills.
- The Demon Swordsman in multiplayer. Instantly discard your opponents hand if you hit them, and is why the lass boss in the Sacred Battle Arena is such a pain.
Surprisingly Improved Sequel: The first game was an interesting concept, while the second game pulled it off surprisingly well.
That One Boss: Helena and The Enchanter in the first game. Both the deities if you don't know their Weaksauce Weakness. And the White Tiger in the first game.
- The Firefox, Beelzebub and Behemoth before the fight with King Leod VIII sure as hell count.