One of Colette's level 2 techs in Tales of Symphonia, Para Ball, only uses 14 TP and is stronger than most level 3 techs.
Don't forget its near-guaranteed hit stun (and ability to stop all spellcasting by itself). She's an absolute monster in the arena for that reason.
Also, once you get the hang of spell canceling, you can perform an essentially infinite combo with Colette if you have an angel tech set to B. Attack twice (non-directional), then quickly press B, then X, then attack twice again, cast and cancel, repeat. This works with any enemy that can stagger, and since the spell doesn't activate, it consumes no TP. Needless to say, it makes the arena a piece of cake.
Sheena can do this, too.
As can any character with access to any sort of spell. That's all of them except Lloyd and Presea, though Genis won't be making much use of it because of his already slow attack speed and strength. However, it's very possible for White Mage Raine to solo the Bonus Boss Abyssion using this tactic, combined with the fact that for some characters, it's ridiculously easy to get immunity to the elements. See here. They tried to fix it in the PS2 Updated Re-release, but it still works if you have a spell you've used 999 times.
Colette's Toss Hammer is a Level 1 Tech that randomly activates instead of Pow Hammer (fairly often) after using the latter 200 times. Easier than it sounds; Pow Hammer is gotten at a very low level, and through the course of battle you'll end up using skills a lot. Alright, so what's so bad about Toss Hammer? The poison damage it does is fast and hits hard, being percentage based- to the point of being able to take out 240,000 hp within a minute and a half. Right here. You can see the HP tick down when he uses a Magic Lens at 01:31. Oh yes, this Colette's at level 6.
It goes even further. Once Pow Hammer has been used 200 times, her Pow Pow Hammer and Hammer Rain skills will randomly include at least one Toss Hammer. If the tech glitch has been used to get Hammer Rain alongside Para Ball...
Lloyd's Devil's Arm quickly becomes infinitely more powerful then any weapon in the game (doing more then 10 times his next most powerful), because the Devil's arm increases when the character makes a kill (even if they haven't found them yet), Lloyd, being the main character, is in the party for all but two short sections and gets the lion's share of kills over the game (and kill count can be carried over for New Game+).
Tales of Vesperia features the exact same premise. Once the player unlocks the true form of the Fell Arms, granted that pretty much everyone will play as Yuri, the Blazor Edge Abyssion will have somewhere in the range of 2500-3000 more attack power than any other weapon in the game for Yuri.
One of Genis's possible EX-Skills is Randomizer (combine Tough and Rhythm). This skill makes his spell-casting times random; every once in a while, the spell will cast instantly. If you control Genis, you can alternate between the B and X buttons (which will repeatedly start and cancel a spell) until the spell casts instantly; this oftentimes makes the spell cast very quickly, even for high level spells, and you don't have to worry about being interrupted in the middle of spellcasting, making the tactic a quick way to bring down even the toughest bosses with a near-constant onslaught of powerful magic.
You can also do this with Raine's EX-Skill Quick Spell (Neutralizer, Spell Save, and Happiness), which causes a spell to cast instantly about 10% of the time.
Sheena has the Hard Hit EX skill, which increases the hitstun caused by her attacks. She also has the Force Seal spell, which also increases hitstun (and stacks with Hard Hit). Finally, her Serpent Seal Pinion has extremely low recovery, to the point that if you have Hard Hit and either cast Force Seal or attack from behind, the enemy's hitstun lasts longer than the recovery from SSP, allowing you to repeat your melee combo > SSP over and over again to keep the enemy perpetually stunned. With this strategy, Sheena can easily solo some bosses like Seles without letting them get off a single attack.
Lloyd has one Tech that lets him keep combos going on his own for as long as he wants. Which one? Demon Fang. That's right, the first Tech he has, is also his most useful. Simply put, it picks up enemies that are Knocked Down, whether due to his own attacks (Technical Knockdown) or due to the number of combo hits (Inevitable Knockdwown). A simple and pretty infamous loop is Demon Fang > Twin Tiger Blade > Repeat. Demon Fang chains into Twin Tiger Blade, which knocks the enemy down. Then you pick them up with Demon Fang and then knock them down again. There are many combo videos and videos of Lloyd taking down the Bonus Boss solo by keeping him locked in this loop.
The sequel also has Marta, who has Speed Cast 1, 2, and 3. Stack all three with some Speed Cast equipment, and she can cast virtually any spell near instantly. Add in one piece of equipment she can get at the start of the final chapter which makes all Artes, spells included, consume only 1 TP (when you're likely to have hit the 999 cap on max TP by this point on a New Game+), and she can single-handedly create an infinite combo by spamming Photon until the enemy keels over.
And don't forget Emil, also from the sequel. With the skills Accelerate (which he could only get from two weapons: his best and a weapon that can be forged as early as chapter three) Glacies, Ability Plus, and Ability Plus 2, he was capable of infinitely comboing anything by simply using Blade Fury (which, due to Glacies has an extra stunning hit at the end) twice, then using any ground based Arcane arte (Sword Rain Alpha and Light Spear Cannon are the most popular), then just repeating the process by using Blade Fury twice again.
And if that seemed too complex, by simply equipping one of the Accelerate weapons, with no other skills, Emil can do an infinite combo with his physical attacks.
Guy from Tales of the Abyss. The game uses a strange combo system for attacks, where using a level 2 or above elemental spell or arte produces a Field of Fonons that was about 25-50% complete. In order to use them, you kept using those attacks until it was 100% formed, and then anybody standing inside it could use an appropriate spell that would absorb the Fonons and make a newer, better spell. Character skills and spells and techniques were customizable with Chambers, one of which (Sunlight) allowed you to use a Field of Fonons before it was complete. Guy had an Arcane arte that created an incomplete Wind Field and also tossed the enemy in to the air. You could immediately combo it in to a Base technique (Void Tempest) which would immediately use the newly made Wind Field to become Dragon Tempest, hitting the enemy before they managed to hit the ground, stunning them AGAIN, which meant you could start the combo up again. This is Guy in action taking on the Bonus Boss starting at one health, never healing, and winning using this tactic.
To elaborate on the combo system: normally it goes normal attack to either a Base arte or Arcane arte, but Base can chain to Arcane. Arcane can't chain to Base though; there's a noticeable delay in comboing, which will kill the combo- unless you get the the skill Super Chain. However, a Fo F skill such as Dragon Tempest can be chained FROM anything and can chain INTO anything... so yeah, there's an infinite combo for you. Let's also throw in the fact that Guy is usually the character to end up with the most speed, and all his techs are fairly rapid already.
Also from Tales of the Abyss is Anise's Ant Lion Man doll. It restores her HP and TP by 8% every 2 seconds or so. On paper, this doesn't look like much, but in practice this is an insane amount of healing. Imagine a mage who never runs out of TP, is a Mighty Glacier, and can only die to a One-Hit Kill thanks to their Healing Factor.
Even more disgusting is the Jewel of Lorelei, which you get automatically through story events, and gives the same benefits to Luke. So you can have two effectively unkillable fighters on the team, one of whom is a combo fiend and frontline fighter. It's a shame that, except for a glitch only found in the original Japanese version, you have to give it up.
Tales of Vesperia includes a marginally weaker version of this in the Final Symbol. It's remarkably easy to gain seven of them.
By using the map glitch, you can get Luke a fancy sword as soon as you can explore the world map. By why stop there? If you saved up enough Gald and carried it over to your new game, you can go to Keterberg and buy the party Mythril weapons and armor, and stock up on gels and bottles. Now you can go outside and fight all the high level monsters (the party will still die a lot, but you stand a better chance with the weapons and armor you just bought). You can get from level ten to level thirty in about an hour (it's even better if you started with an EX Px 2 or EX Px 10).
Jade has his fonic powers sealed due to plot reasons, and he takes a while to build back up and unlock his various spells. If you give him a Capacity Core that boosts the Enhancement stat, he will eventually learn the AD Skill "Glory". Glory makes a character completely immune to stagger/flinching. Jade will then be able wreak havoc from afar, tossing out spells left and right with no fear of his incantations being interrupted. Tear can do something similar. Her offensive spell selection is much more limited, but Holy Lance is a very good spell.
Even Natalia is great beyond all degree. With the right set-up, she becomes a beast; already the second fastest AND the second most durable, along with having a very effective ranged move, which is repeatable, it's very easy to make the princess an unbeatable, untouchable Goddess of death. If an enemy gets close? She has quite a few moves to keep the distance. Or, she can run. Incredibly fast. That's not even going into her Mystic Artes...she's one of only three characters who has an UNDODGABLE mystic arte, and her second is one of the most damaging in the game. Add to the fact she has healing magic, her artes are VERY easily reusable, and her physical power is ironically really good, she could solo the bonus boss just as easily.
Rita in Tales of Vesperia. Get the Spell End and OVL Boost 2 skills, equip the Risky Ring, shortcut Tidal Wave to the A button, set all of your other characters to taunt. Go into battle, launch the enemies into the air with a single level Overlimit, mash A. Have fun. And don't forget to use another overlimit when the overlimit ends. Alternatively, you can also use the Minimum Damage skill along with it to farm Grade off of enemies like no one's business.
Wonder trinket allows you to fit in even more tidal waves. Violent Pain is used the same way on single foes, to maintain permanent overlimit.
Estelle is the real game breaker. Due to an unintended oversight (which was fixed in the updated PS3 release), the spell Force Field makes the target invincible for a few seconds, but with the Eternal Support skill, there's a chance it'll last for the ENTIRE BATTLE. Put the skill on, spam Force Field until it sticks, and you can't lose.
You have to work a bit to get Yuri's infinite combo going, it requires the enemy to be stunned (with the spinning stars) early on so they can't break out of it, and it only works against solo enemies, but once you get the hang of it and remember to reset the fallen enemy with his free-run slash, Yuri becomes impossible to stop, especially against human enemies. There are videos of Alexei,Flynn,Duke, and Kratos all being brought to their knees by an infinitely-comboing Yuri.
The PS3 version makes Yuri even more powerful with the addition of his Vesperia No. 2 weapon, which gives him a second Mystic Arte, Tenshou Kouyokuken. With the Combination skills, Yuri can build up a 100x multiplier to be applied to the next attack that hits the enemy. Tenshou Kouyokuken is one of the strongest single-hit attacks in the game, probably second only to Indignation. Throw in the Barbatos Ring, which gives an incredible stat boost at the cost of not being able to use items, and painful destruction ensues - imagine One Hit KOingevery boss in the game.
It's possible to infinitely time stop anything in the game without much effort, completely removing any difficulty whatsoever. All one needs to do is get Raven's Stop Flow spell(Stops time for 5 seconds with a long cast time and high TP cost), equip him with the Risky Ring(Makes all Artes cost 1 TP), activate Overlimit 1 for instant cast time, and beat anything on the screen to death. With minor use of limit bottles, it's possible to maintain this technique for essentially forever.
While not exactly useful against bosses, assigning Sylph and Arche's weaker spells to your hotkeys basically ensures an instant win against the incredibly powerful monsters in Shadow's cave. Being able to cast high level spells much faster than your characters can is not as useful as you'd think when the other guy is constantly knocking you out of your casting with a puny little ice needle. Of course, everyone should thank Dhaos for showing it to us with Tetra Spell, which basically fires all three of the weakest spells in the game at you at once, knocking you off balance for the truly epic ass-kicking he's following it up with.
In Tales of Rebirth, it's possible for one with enough time and knowledge of the enhance/inherit system for equipment to make a weapon that makes the wielder cause 32 times more damage.
In Tales of Xillia, all elemental damage has a chance to inflict status effects on enemies. Light-element attacks have the chance to inflict the Confused status, which makes a combatant attack their allies. Milla gets a powerful Light element spell late in the game, a little bit before she can pick up the Aerial Spell skill. With her Double Jump, Aerial Spell is a game breaker on its own, since Milla can jump too high for enemy AIs to detect and attack her, and Aerial Spell keeps her floating there until the spell resolves. But once you boost her chances of inflicting status effects on enemies (usually by boosting her Intelligence stat), Milla is an absolute monstrosity against any group of multiple opponents, as she can get them to kill each other with ease. What's worse is that most of the powerful boss fights in the storyline (including the final battle against Gaius and Muzet) have them come at you in groups. Difficult boss battles become hilariously trivial when you can confuse them into using their Mystic Artes on one another.
For a two-player tactic, Rowen and Leia linking together can use the Linked Arte Stop Flow (Stops time for a few seconds). Have one player distract whatever you're fighting and the second do nothing but cast Tightening Chains while paired with Leia, and depending on how many hits player one can get in the Overlimit bar will always be high enough to at least start casting another Stop Flow before the first one ends. With careful timing nothing but player 1 will ever be able to move again.
In Tales of Xillia 2, Ludger's signature Chromatus mode is absurdly powerful, as it allows for combos that are only limited by the duration of the mode itself (Which is extremely long once you get all of the abilities that extend it), but most notably, maintains the damage bonus you get from exploiting an enemy's weakness for as long as you can maintain the combo, meaning it's entirely possible to almost a minute of time to exploit a 5X damage multiplier when it would normally fade away after a few seconds. Not even the strongest bosses in the game can live through it if you can activate it while the damage bonus is at its max.
The Combo Spell ability is another extremely potent ability, as it reduces the character's casting time proportionate to the number of hits they've scored. This basically means that characters with it can keep casting spells back to back to gradually decrease their cast time until everything they use triggers instantly, allowing you to completely fill the battlefield with spells that can keep even bosses locked down indefinitely due to the status ailments some of them can inflict.
While they start off as Boring but Practical skills, Assault Step and Mirage Step become this when used together. Assault Step turns your forward dash into an attack that does damage. Mirage Step lets you dash forward and back without using AC and makes it possible to get closer to the enemy quickly just by dashing repeatedly. Considering how you can cancel an attack with a dash, these two skills basically give you a free way to extend your combos and AC usage.
The Director's Cut version of Tales of Destiny Remake takes Leon's Demon's Lance spell from the original and buffs it to amazing levels, even renaming it "Demon's Lance: Zero". He's hard to stagger out of casting, and on release it fires a massive, screen-filling attack and also grants him immunity to the next seven hits.
Leon also has access to the sword arte "Marian...", which fires a sword beam, creates an energy aura, and makes him immune to all attacks while he's using it.
Leon's Chaltier also has the highest critical chance of any Swordian (plus a chance to petrify) in a game where criticals restore CC, letting you continue your combo as long as you keep getting them.
Johnny doesn't gain his Maware Rondo until fairly high level but it allows him to easily stunlock many bosses including Bonus Boss Barbatos who is weak to its sound element.
In Tales of Destiny 2, magic in general is usually this, due to how much more damage it inflicts compared to melee, but special mention goes to Harold's full Crazy Comet hi-ougi chain. It's usually a One-Hit KO, dealing much more damage than any other attack in the game (people only showcase it on bosses because it seems to kill everything else too quickly) and healing your party. It's really only balanced by how much TP it consumes to cast, but since items don't have a time limit like in several other Tales Games nothing's stopping you from just getting all the other characters to heal her TP. Worse, it didn't even count as a true hi-ougi in this game (this was fixed in later games with the spell) so Harold doesn't have to go into overlimit to use it.
Harold at high levels is incredibly powerful against solo bosses, able to combo-lock them with either speed-cast Divine Saber spam (her highest-level Light spell) or her base melee techniques Raigashou (Light elemental) or Chouishou (Dark elemental), as thesevideosshow.
Tales of Graces on the Wii gave Hubert a combo capable of one-shotting the giant Bonus Boss Rockgagong, netting the party around 30 levels in one shot. Patched out in the PS3 version.
The Killer attribute (which deals additional damage to humanoids, including many bosses) can be obtained early by fusing Richard's Steel Sword with a Darkshine Crystal to create the Fame and Faith sword, then transferring the quality onto a gem anyone can use. This is highly advantageous as the Killer attribute is otherwise very hard to obtain and hitting weaknesses is vital on higher difficulties.
While it requires the late-game Accel ability from the epilogue arc there is a way to basically one-shot any boss, including the bonus bosses. Equip a "Boost Next Strike By Damage Taken" title, use books and difficulty settings to raise the damage you take as high as possible, then activate Accel the very instant the boss hits you. Thanks to Accel you won't die, and the next strike will one-shot the boss.
The Accel ability in general qualifies, since activating it fully restores your CC (action points), allowing for much longer combos) and instantly casts any spell that was charging when it went off. That's even without mentioning the character-specific bonuses while it's active, especially Cheria's which stops time. And that's without using costume titles to have four Cherias running around...
Tales of Eternia: Max equipped with the Canceler accessory is beyond belief. The Canceler gives him the ability to charge a weak spell, then cancel the spell and cast a high level one in its place. His best skill, Elemental Master, counts as a high level spell with no other requirements other than a long charge time (and a long sidequest to get it). Elemental Master covers the screen and shoots around sixteen laser beams. Have fun spamming it.
Even without using Canceler, Max has the Aqua Spiral tech which is obtained fairly early, hits only a single target but does absolutely insane damage (slightly above Elemental Master, even) and is fairly easy to use (unlike the slightly stronger Rage Laser which requires specific spacing between Max and his targets). Most bosses are destroyed with just a few casting of this, even the extremely powerful bonus boss Sekundes.