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.
Anticlimax Boss: Coming right after Bloodbane, Fenrir can be this because of his weakness to fire. Infernas, a sword you pick up much earlier in the game, can kill him in one hit.
Awesome but Impractical: Aelia's Purify Weird Soul, Dreaded Dragon, looks cool, but it doesn't do a lot of damage or build up the meter very much. The Purify Weird Souls of a few other characters fall prey to this trope, as well.
Boring but Practical: Because the time in which most party members are recruited is based off of a mixture of random chance and fixed events, it's not uncommon for players to have their endgame party established as early as chapter four. Because the player starts out with Arngrim, and always recruits Lawfer and Aelia (All three of which are very common choices) in chapters two and four respectively, by chapter five, you'll pretty much only be swapping them out to train someone to send to Valhalla.
Complete Monster: Lezard Valeth is the heroine's Stalker with a Crush with a twisted, self-absorbed idea of love. In order to lure the valkyrie Lenneth to him, Lezard seeks out his former magic teacher and turns her husband into a monster so he murders her, gloating that at least they have a "lifetime of love" behind them to have enjoyed. Pursuing Lezard, Lenneth discovers a factory he's created of homunculi, kidnapping beings and experimenting on them to remove their souls and create his supply. After this, he promptly kills someone who was closing in on his secrets by freezing her body when she's astral projecting, trapping her spirit outside. In the second game, Lezard manipulates all of time, devours Odin himself and rips out Lenneth's soul to fuse it with his own and remake the world in his own image as the supreme creator.
Banshees have a good chance to appear early and can wipe out everyone (including your rear caster) with just a couple of their AOE attacks; good luck if you didn't get Nanami and her Dragonbane in the second chapter.
Mandragoras in the Forest of Spirits, which can wipe out even a well-prepared party with a single multi-hit AOE attack. And they come in groups. Most gameplay guides will advise the player to run away rather than fight— and not without good reason.
The various Eye enemies (Evil Eye, Inferior Eye, etc.), who deal high damage, take a ton of hits to defeat, lack weaknesses, and can revive their fallen comrades. On Hard Mode and Asgard Hill in the A Ending, they regularly pop up in groups of two to three.
Many others appear in later dungeons, which is one reason you want to make good use of those "Slayer" weapons.
Hamsters. They randomly show up in place of Loki Shades in the Seraphic Gate, and will give you a violent phobia of cute furry things in short order.
The fully leveled skills "Guts" and "Auto Item" in combination make it almost impossible for the entire party to die, rendering other skills of the same type basically pointless. Add some Angel Curio's and you can survive anything long enough to win. This technique becomes necessary against Bloodbane and the Bonus Dungeon.
The "Reverie" skill can also boost both attack power and the amount of EXP gems you pick up by adding hits to the combo. Using it well can make most fights a lot easier, and level up your characters a lot faster as well. The best part, though? You can use both sets of game breakers together.
Goddamn Bats: Any of the slime type enemies. They're generally no real threat to you, but they come in groups of two-to-three, and they're so small that most of your characters' attacks will go right over their heads, even more when they're knocked down, as will a good chunk of magic attacks, so they're tedious at best to fight.
Porting Disaster: Well, not quite a disaster, but the original US release for the PlayStation had some notable improvements over the original Japanese version, generally streamlining the menu and organization of your characters and items. The PSP rerelease, however, was based on the Japanese version, and thus doesn't have the improvements from the US PlayStation original.
The Scrappy: Llewelyn's high voice and whiny attitude typically gets him sent off to Valhalla ASAP. It also doesn't help that his attacks aren't very accurate.
The Beholder-like Hel Servants in the Dark Tower of Xervah, a Hard Mode-only dungeon. They are extremely durable, hit hard, and can bring each other back from the dead if you don't kill them off at the same time. Plus, you likely don't have any of the really good equipment at this point, since they can only be accessed if you have a certain item... and the Servants guard the first prerequisite to that item. They later reappear in several other dungeons as Demonic Spiders.
Any late-game boss that can perform the same Disc One Nuke Great Magic spells mages in your party can potentially learn is this if one doesn't have the proper items equipped. (And sometimes, even if one does have them equipped, they still are!)
The Wraith and Akhetamen are hard unless you have Holy Water of Mithra because they like to spam Gravity Blessing or Seraphic Law respectively every 2 turns. The latter also heals himself to full if you brought him to 20% HP, so you have to hit him a couple of times before tossing the Holy Water at him.
Bloodbane in particular is quite egregious. If you can beat him, you should have no problem with Surt, Fenrir, or Loki.
Lezard Valeth's tower and the Tombs of Amenti are Hel for their sheer length.
The Celestial Castle. If you fall down in the outdoor area, you will be sent back to the world map, losing 2 periods. And did anyone mentioned that those chests on the platforms all have explosive traps on them? Also, get into a fight mid-jump, or while going across the chains, and you're in deep shit once the fight's over since you'll just fall straight down.
About half the players hate Llewelyn because of how he is in-story; the other half hate his terrible attack power and tendency to miss. His only saving grace is that using his finisher guarantees the chance to use another finisher with a large combo attached to it - and there are other characters who can do it nearly as well.
Grey's only saving grace is his high defense; otherwise, his attack power is mostly poor, lands few hits, and he has a mediocre Purify Weird Soul attack.
Kashell has the weakest Purify Weird Soul in the game. Granted, he actually does have better attacks than Arngrim.
Badrach's attacks are weak and can only connect well with larger enemies. To make matters worse, he has the lowest hero value out of all the Einherjar, so sending him up to Valhalla (which many players often do) is difficult. An archer is the needed Einherjar for Chapter 5 (one of two chapters you can get him in), so that only stacks up the difficulty.
Archers in general - unless they're named Janus or Lenneth with bows.
Sorcerers/Sorceresses often fall into this but it's mostly because you only need one, and most people pick with Mystina or have picked their sorcerer far earlier in the game. This would be okay if the later chapters didn't throw a lot of them at you. Mystina is one of the better sorceresses, besting Jelanda and the three sorceresses from Yamato, and is equal to Lorenta (obtained earlier, and necessary for the A ending) but her power pales against Lyseria and Gandar (both only available on hard mode).
Because of the Disc One Nuke nature, some characters like Jun, Suo, and Jayle may fall into this simply because they're Overshadowed by Awesome. Suo in particular can only be recruited in Chapter 7 - when a player will pretty much have their endgame team long established.
The Woobie: As it's a game about people before their final moments, it has a few.
Celia. The last surviving member of a mercenary group that included three of your eventual Einherjar, and the only one still alive at the end of the game, and the last you see of her is her breaking down when the last one dies. Made worse by that recruitment scene looking, up until the death, that it is hers. By the end of it you want Lenneth to pop in and just give the poor girl a hug...or a stab in the stomach.
Shiho was born blind, forced to become a song maiden for her nation, abused by her military superiors and even her fellow townspeople, and to top it all off, she was killed right after meeting the one man who acknowledged her as a person and not just as a tool for battle.
Most notably, the change of Ahly's name to Hrist; Hrist was a valkyrie in Norse mythology.
For whatever reason, Freya is named Frey in the Japanese version, which is the name of her brother, despite obviously being the Norse goddess of love and fertility. Strangely enough, there is a character named Frey, but it is a young girl named Frey who is the sister of Freya. She is named Freya in the Japanese version for some reason.