These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anti-Climax Boss: Gaia's second form, as it only attacks the party with low-level spells like Snooze and can be easily bested with normal attacks.
Game Breaker: The ultimate attacks of Justin, Feena, and Rapp (Heaven and Earth Cut, End of the World, and Neo Demon Ball, respectively), ESPECIALLY when used all at once.
Justin is a one-man army full of incredibly powerful special moves, has access to the best equipment, can make himself invincible, amazing stats, and a wide variety of magic only surpassed by Feena and Liete. All for the small price of using axes (which grant strength and HP boosts) and maces (which grant HP and defense boosts). His Dragon Cut is also not only a very powerful attack that hits all enemies, but it makes training his weapons, Fire and Earth magic a breeze.
Although Sue doesn't remain in your group for long, you can have some fun with her! Give her Fire and Earth ASAP, and learn her "Fight!" Cheer (it is the only Fire+Earth+Mace skill she has). Spam this over and over and over again (since the skill is used on allies and not enemies, it always skills up the same amount, and isn't subject to diminishing returns like offensive skills are) to end up with a Sue (Mary Sue?) that can one-shot nearly anything. Since "Fight!" Cheer levels Fire, Earth, and Mace skills, she gets all of the stat bonuses from that, plus the added attack power of having a ridiculously high weapon skill. Also, her Defense and HP will be so high, that nothing will actually be capable of KOing her until she leaves your party.note Which actually makes it a case of Gameplay and Story Segregation, given her reason for leaving...