From the game:
- Game Breaker: Some of the storyline and secret cards have ridiculously useful abilities, like "increase all allies HP max".
- Cait Sith is a good example. Immune to spell cards and can't be damaged by creatures with more than a certain base attack. Set up on a max level fire space, only a few card can even stand a chance of killing it, though a scroll attack (which penetrates the defensive land bonus) will kill it in one hit.