Fang Wolf. He uses repeated combo attacks that can instantly floor even the Mighty Glacier.
Undine. She wouldn't have been so challenging if it wasn't for her wave attack. Her wave attack would run from right to left across the screen and would hit every one of your party members and had a very small charge time. The wave would hit for somewhere in between 600 and 700 damage to your entire party when, at that point in the game, you'll be lucky if anyone other than Cless has over 1300 HP. What really seals the deal for being absurd is that Undine is capable of spamming her wave attack. If she starts spamming it, then there's nothing you can do to survive. It also doesn't help that she usually casts Delay on Cless so it becomes even harder for you to interrupt her.
Many summons are like this, but Undine and Celsius stick out in particular. Undine casts "Spear of Baptism", a move that pierces across the entire battlefield. It has to charge for a couple of seconds, which would be great—except that Undine is also untouchable while charging, which neatly nixes any attack chain you have going. Celsius, meanwhile, is extremely agile, can inflict Freeze with a touch, likes to sidestep physical attacks, and will cast Blizzard, one of the most powerful Ice spells in the game, twice in rapid succession, which can easily kill an otherwise appropriately leveled party with no room for argument. Oh and let's not forget Sekundes who can cast Maxwell Extensions. Consecutively.
The battle against Alice and Decus near the end of the game. First of all, due to a plot event all of the original Symphonia characters can't help you - it's just Emil, Marta, and your mons, which if you haven't been raising worthwhile mons (which you likely haven't been if this is your first run) effectively means you're two characters short in this battle. But that's just the start of it. Decus will constantly go after Marta with his BFS, making it just about impossible for her to do anything without Emil defending her...except that this means Alice gets her spells off, either healing Decus for ridiculous amounts or dealing ridiculous amounts of damage to your characters. Alice has amazing defense for a spellcaster, and Emil going after Alice means Marta will die in seconds. And both of them have an attack that instantly reduces the HP of your entire party to 1, is unblockable, and is impossible to move away from once it starts from completely out of nowhere...very often resulting in this.
There's also the Lookins. The fact that their role in the story is as packhorses for the Vanguard that you've been beating senseless throughout the whole game does not help. These things have an extremely fast rolling attack - fast enough to cross the entire battlefield in half a second. And they will be all too happy to spam this, as it's the only attack in their arsenal. You can't combo them as they'll wind up rolling toward another party member, and since in this game getting hit while free running is a guaranteed critical, their attacks will be very damaging. And there's FOUR of them.
Stingle. Crazy attack power, high HP, really long reach and (get this): he can enter stances that if you attack him, he'll counter-attack. This will hit everyone in range. ONE OF THESE IS AN INSTANT-DEATH SKILL. Which means that if you're not paying attention you can have three quarters of your party dead in an instant and not know why. Luckily the preceding boss drops an anti-One hit KO item, but if you didn't know this beforehand, you're dead.
Dark Senel and Dark Shirley deserve a few mentions. The former distracts you while the latter casts her un-dodgeable Blizzard spell, which can kill the entire party.
And speaking of dark bosses, Dark Chloe is a nightmare. Not only does she make use of the actual Chloe's skills (and recall that Chloe is this game's sword user, meaning she has a variety of useful skills that cover all sorts of impressive ranges) and hit like a tank on steroids, but she is nearly IMPOSSIBLE to stagger, so she'll often charge right through any attacks you attempt on her and combo whoever the hell she wants to death. On the plus side, she's vulnerable to the Bind status, which temporarily immobilizes the target. The problem? The only way to inflict Bind is through one of Chloe's Compound Eres, which A. You may not have as your current Compound Ere, and B. The computer NEVER EVER USES COMPOUND ERES, so if you're not controlling Chloe yourself (and unlike other Tales games, you can't switch characters mid-battle in Legendia), or unused to using her Compound Eres, which are all counter attacks, you're still good and screwed.
The Gigant at the end of the Ruins of Frozen Light is cited as one of the most difficult boss fights in the game. It has ridiculously high health and incredibly high attack strength, so not only does it hit really hard, it'll take a long time to kill. It's significantly harder than anything fought up until that point, and has a well-earned reputation for being one of the most frustrating bosses in the game.
The ostensible Final Boss that resides in the (supposedly) Very Definitely Final Dungeon: "Master" Van. It was a climactic point in the story and was built up to be the final confrontation before the Your Princess Is in Another Castle, yes, but the battle is still far and away the most difficult you've had so far—none of the other bosses up to this point even came close. Over 100,000 HP, throwing out high-level Arcane Artes like breadcrumbs, and only the second boss so far to use a Mystic Arte that can, and will, instantly kill anyone who's caught in the blast radius early enough in the attack.
Arietta, when you fight her at Choral Castle, is one of the nastiest bosses in the game. She has two monsters helping her out which are more than capable of killing your party, and that's before you factor in Arietta herself buffing them and casting dangerous attack spells. Want to try and stop her from doing so? You'll need an ally to help you attack her, otherwise she'll just shrug off your attacks and cast the spell anyway, and good luck actually getting your computer-controlled allies to not be idiots and attack her. This is assuming she doesn't go into Overlimit, which prevents hitstun even if you do get an ally to join you. If you're facing her with New Game+ benefits on a higher difficulty, she gains Mystic Artes too. And this is the second boss of the game.
The final boss, King Aurella, actually puts up a fight: he hits hard, teleports around the battlefield, and attacks with several status ailments. This can take some players by surprise, because up until that point, every single battle could be easily won by mashing the A button. He's still not quite tough enough to be considered hard (especially because by the time you get there, you should have a lot of healing items), but compared to the rest of the game it's a huge difficulty spike.
Cerberus. He's got several attacks that almost always miss, but he also frequently does a diving kick that chains into Grave/Stalagmite, which is very, very hard to dodge if it's aimed at you. However, he comes with a pair of dogs that, while they don't have any techniques at all, are very fast, have combos that will always interrupt your attacks and send you flying on a low enough trajectory to make it very, very unlikely that you'll pull off an aerial recovery, and have only slightly less HP than Dog Boy himself. Oh, and it's almost impossible to stagger them, in a game that pretty much requires staggering to pull off any kind of damaging combo and/or stop your enemies from wiping the floor with you. The only good thing to come out of this fight is that your last party member joins right after it, and their fighting style is pretty much exactly the same as Cerberus', albeit with the expected basic-techniques-only moveset.
The Gigantess Omega. Its kicks, dashes and shots deal insane damage to the party, but to make things worse, its tornado is pretty much guaranteed to KO anyone near him who weren't quick enough to guard or move out of the way. Oh, but that's not even it's trump card: it has an energy surge attack that knocks down and away anyone near it (i.e:the melee fighters, who are the #1 powerhouses) while dealing nigh-OHKO damage at the same time. And it uses it without any warning. Did we mention that it's constantly on the move, rarely touches the ground and never stays in one spot for more than a few seconds?
Ange and Albert in Tenos. A Dual Boss where one is a fast and powerful melee fighter who can also heal, and the other is a ranged attacker who's constantly interrupting your team and can cause Standard Status Effects. And as an added bonus, you're short the party member who was probably your main healer.
Gattuso in the Ehmead Hills. Unlike the previous bosses, Gattuso is much higher level, has much more HP, more attack power, is much faster, and can poison you with its melee attacks. To rub it in your face, immediately afterwards the party talks about how easy the fight was (or at least Karol does), and the next boss is a freaking cakewalk. While it's not necessarily the hardest boss fight in the game, it just comes way too early for most players and gives them the real first taste of the boss brutality to come. Hence his nickname, "the Noob Killer".
Captain Schwann. Your party is missing its two primary healers: Estelle (who's being held captive by Alexei) and Raven (who is the boss you're fighting), and he will take full advantage of that with devastating strike artes and a hard-hitting Mystic Arte that's likely following the killer combo he just pulled on your party since he was in Over Limit.
Khroma has an incredibly damaging tail attack she just pulls out of nowhere, and can fly around the battlefield, forcing you to chase her down. The worst part about Khroma is definitely that attack when she sends multiple homing fireballs at you. It's nigh unavoidable and sometimes causes Sealed Artes.
Alexei, because aside from being really strong, he'll spam his Brilliant Cataclysm Mystic Arte which he can literally pull out of ANYWHERE. In the middle of comboing him? Bam, Brilliant Cataclysm. He can even use it when he's not in Hypermode (which is the requirement for Mystic Arte usage) and the move is completely undodgeable and hits like a truck.
Clint has an amazing Mystic Arte which is a Fatal Strike. Thats it. He Fatal Strikes anyone who has the symbol on them. Whole party has it? Gameover.
In the PS3 version, to some extent, Duke's second form and his third form. Unlike the Xbox 360 version, Big Bang can kill, instead of leaving everyone with 1 HP. Since it deals thousands of damage on all difficulties, you're basically looking at an easy one-hit KO on everyone. Better pack Rebirth Dolls and have that Ressurect skill ready.
Kornerupine the mad scientist and Incarose-ILL. All that needs be said about Cornelpin is that, until names began catching on, he was known on virtually all message boards, videos, and other American fan works as "that fat bastard with the spin attack". The latter is a retread of a previous Hopeless Boss Fight with more defense, more combos, bigger spells, a Hi-Ougi, and in general roughly as much power relative to the player owing to sheer annoyingness. And you're supposed to win.
The Slime Queen. The queen itself is not that hard, but the slime enemies she summons can poison your characters and easily gang up on you if you're not prepared. Since a lot of the boss fights in this game are Flunky Bosses that can inflict Standard Status Effects, the Slime Queen also serves as a Wake-Up Call Boss.
Kurt. Oh Kurt, you have tens of thousands of health and constantly summon more mooks, It doesn't help that Kurt has a VERY large range of attack and will go right for Cheria and Sophie (unless Malik is in the fight, in which Kurt will focus on him). And let's not forget that he can guard so quickly that your attacks will almost never deal full damage, while he throws around guard-breaking attacks like it's going out of style.
In Tales Of Graces F (the PS3 port) there is a boss that you fight in the events of the game's story after the ending of the Wii version. This boss is Little Queen. Or rather, it's your party against a total of TWELVE Little Queens. They aren't all out on the screen at once, probably only because the game would experience slow down. Small comfort that is. The problem with this fight and said problem escalates in the second battle against her the boss can do your characters in in one hit(combo in the case of the physical attacking one, there are two other versions, one with nothing but projectiles and another with magic.), two to three if you're lucky and have amazing equips on Evil or Chaos difficulty. God help you if you bought DMG x5 or x10 in the grade shop. The insanity of this fight escalates the second time you fight said boss, because in the previous one, when you kill one, another takes its place until you beat all twelve. The second time? You kill one, TWO appear to take its place. This just leads to the fight being seemingly impossible due to not even being able to attack, because if you do, 2 or 3 of them will just RIP AND TEAR you from the side or behind; and in the second round, they have Blast Calibers too. Also they can seemingly ignore Cheria stopping time — at least their bullets, spells, and Blast Calibers love to. Also, if you're unlucky enough to kill two or three of them at a time... let's just say it won't be pretty.
The Duel Boss fight against Muzét. She can hit you with pretty much every status effect in the game, and you don't have a healer to save your ass in this fight. Equip an Amulet to guard against the status effects? Her Artes can still burn (constant damage over time) and slow you down, on top of dealing a ton of damage... which just means that she can continue spamming her Artes on you to wipe the floor with you. Managed to dodge it? She has a really fast attack that strikes in all directions and knocks you down. Blocking constantly? She has an unblockable attack that can easily take off a third of your total health. Let her get in a hit, and she can hit you with an unescapable combo. She also has high-tier spells of pretty much every single element, has a crapton of HP and has such a high defense stat that you will only deal damage in the 3-digits, even with high-powered Devil's Arms.
The fight against Victor can be downright murderous compared to any of the previous bosses. He essentially takes everything that makes Ludger deadly and turns it against you (Including his Super Mode, which gives him access to an additional Mystic Arte for extra chances at a Total Party Kill). He's extremely quick and aggressive, being capable of tearing through the party in seconds if he's not kept under control, and anyone who does survive one of his combos typically is left with multiple status effects that will either remove the remainder of their HP or render them near useless. He's also smart enough to start spamming guard breaking artes if you try to block him, and has an instantaneous healing arte that restores about 15% of his HP whenever it's used, and he can do it as much as he wants. On top of this, he doesn't have any elemental weaknesses, making the Might Charge 3 ability your only way to get a proper combo started on him outside of a lucky guard break, and as a last bit of icing on the cake, you're forced to use a party formation that lacks any of the characters who could be considered proper healers, which makes staying alive even more difficult.