Demonic Spiders: The Vodianoi. They go down in one hit from an Earth card, but without them, you're screwed.
Ear Worm: The first game's soundtrack is riddled with these. Shame there's no OST for sale.
Game Breaker: You pretty much needed them for the first game, or you'd be stuck at the God of Destruction, and you can miss them too. The second game dulled those cards down, but now arise cheap methods to beat both fights with the God of Harmony (see Weaksauce Weakness below).
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.
Surprisingly Improved Sequel: The first game was an interesting concept, while the second game pulled it off surprisingly well.
Tear Jerker: Helena's ultimate death in the first game. Sol in the bad ending.
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 & Behemoth before the fight with King Leod {=VIII=} sure as hell count.
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