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: Rogue. Despite being played up as the Bigger Bad of the game, it goes down quickly. He uses a multi-hit attack that deals 1800-2000 damage to each party member... but has to charge up every other turn. As long as you spirit when he attacks to reduce the damage to a paltry 800 or so and use the off turns to heal, you can pummel him with minimal effort.
Pueras, thanks to their "Death Game" ability, which has the ability to instantly kill itself. Or you. Hope you were paying attention to the cards.
Vera seru. They hurt-a-plenty (they do MORE damage with their special attack if you Spirit unlike most other enemies), are somewhat fast and they heal themselves. Not particularly hard, but they can be annoying, especially if they nail Noa with Bloodsucker while she's Spiriting. Oh, and they look somewhat like bats too.
Memetic Badass: Gala, thanks to the scene with Juggernaut and his ability to Fly, as shown in Sol Tower.
Most bosses aren't particularly difficult if you have enough levels and remember to guard and heal as needed. The two bosses that stand out are the Berserker and Xain.
The Berserker has extremely high attack at this point in the game and uses the Rot status effect, which makes it very difficult to attack and use Arts. Berserker also is the second boss in the game with a very hard hitting spell without requiring a turn-up to charge for the spell. The only thing that may help you is that there's an amulet near the boss that prevents rotting.
Xain requires that you've learned the Orb spell (a heal spell for all allies) and that you guard when he charges up his Terio Punch. Proper battle tactics will take them both down without too much trouble.
If you just go plwing the enemies without strategy, Zeto, Dohati, Neo Saryu, Gaza and the Delillas family will DESTROY you without much trouble.
The Woobie - All three main characters have depressing events in their backstories, but Noa, who, among other things, finds her real parents only to help cause their deaths very shortly after (they knew what they were telling her to do, she didn't), finds her brother only to discover he's the Big Bad, and loses the closest thing she's got to a parent at the very end of the game, takes the cake.
Legaia II: Duel Saga:
Game Breaker: Full Power. This skill activates when you guard and gives you one fewer Art Block. In return, your character's STR triples for the following attack. Since damage is based on (STR + ATK - enemy DEF), this generally means a 5-6x increase in damage. Combined with Mystic or Variable arts and a reasonable level, it is very possible to defeat the final boss in one attack sequence.
That One Boss: Bubba. To be more specific; Bubba in the Arena. Bubba is armed with arts that can Charm (Violet 7), Stun (Final Bullet), hit really hard multiple times (Dancing Revolver), or worse, regain all of his HP (Golden Memory, though he only does this if you put the ring on Elfin's Grave), he can prematurely end a streak if the player isn't prepared to deal with him.