Capturing monsters. By throwing a monster-type enemy (not a human-type, unfortunately), it will join your party under certain circumstances. Seeing as throwing an enemy on one another fuses them together, adding their levels together, it takes only 8 hours to go from a level 500 character to five level 9999 characters.
No longer possible in the games following Disgaea 2 (The remake of 2 included), as you can't capture monsters of a higher level than the main character.
Even as late as D4 you can utterly break monster captures, with the help of the Land of Carnage. The level cap for the games is 9999, but LOC enemies continue to gain stat boosts even beyond that. Their official level may be 9999, but they are actually much more powerful, and if your own characters are 9999 (pretty much a certainty if you are in the LOC) then you can capture them. You just need to be clever to do it. There is an innocent that can render you immune to physical attacks on odd turns, and magical attacks on even turns. So simply determine which type an enemy will use, make sure one of your characters has that innocent, and attempt a capture on the proper turn. You are guaranteed to win.
The capture mechanics differ again in D5. While you can use a variety of skills to imprison an enemy and later conscript them, you have to unlock their class to do so, and you have to be their level or higher. The same cannot be said out of using them for enhancing your Squads or making Extracts, however.
It's possible to apply both the poison and sleep status ailments to any non-boss enemy to kill them off with ease, regardless of the level difference between you and them. As a result of this, one can clear some optional maps with extremely good rewards (Dark Sun level 1-4 of Axel Mode in Dark Hero Days in particular) far earlier than intended, and break the game with the absurdly powerful equipment and tremendous EXP rewards from them.
Magic Knights tend to be in the running for the best class in multiple games, outside of very few specific exceptions. In essence, take Flonne from the first game, give her more survivability and an innate ability that increases the damage of elemental spells and you have the Magic Knight class. If you want to push it further, have them have an innate resistance to their element since their second ability, Elemental Force, lets them further increase the damage of the element spell they're strong against. Their unlock conditions are usually really simple too, usually requiring you to level up a Warrior/Valkyrie and Skull/Mage up to level 25. Once you unlock them, make three that specialize in each element, teach them the star and heal spells and watch them proceed to tear the game a new one.
On that note, spell users, in general, tend to be miles ahead of other classes when it comes to the sheer raw damage they can deal out at a surprisingly far range. Gunners and bow users often pale in comparison to the range spell users can obtain after even a slight amount of work put into them, with far higher damage output than both the previous classes combined, on top of spell users getting AoE modifiers to their spells to hit several enemies at once; often way more than most other AoE attacks can hit. The things that are supposed to balance them out, elemental resistances, immunities, and paper thin defense are easy to work around or ultimately don't matter; just have your mage/skull who specializes in the element the enemy is weak to deal with them or give them the star line of spells to nuke them for Non-Elemental damage and, after you reach a certain point in most Disgaea games, defense onanyunit doesn't matter anymore. Often, the most 'efficient' party in most games consist of three mages/skulls specializing in each element, a few fodder units to draw agro off them and maybe a storyline character or two.
Violence, an evility that first appeared in Disgaea 3 and would continue to appear afterwards, boosts a unit's stats by 50% while decreasing their EXP gain by 50%. At level 9999, it's just a free, massive stat boost that puts any other to shame. 5 nerfed it severely by causing it to take up 5 evility slots, making it less efficient (but still nice to have).
The More Expensive Stuff proposal at the Dark Senate, if one is lucky (or just save scums), can open access to the best equipment in the game very easily, combined with the extortion option and other easy money schemes can cause your level one character no rebirths to be on tier with level 50 characters with ease, at a point in the game where enemies are only level 20.
Flonne, she starts weak, but if you give her pupils to learn all magics, give her a bow and she can nuke anyone from across the map.
And considering that a Galaxy Mage/Skull knows pretty much every spell bar defensive buffs, you can have all her other pupils beef up her other stats.
The Yoshitsuna sword makes all the other weapon types useless in the first game. It has more range than a gun, higher Attack power and capability than any other item PLUS stats boosts in all areas and, coupled with the sheer variety of skills that swords provide, it can cover almost all the area effects you need. It was considerably nerfed down in the sequel.
The DS remake of the first game makes Pleinair a playable character. She has great starting stats, good stat growth, 2 counters, good movement, and above average aptitude for every stat except Defense, which isn't important because she has a 150% speed aptitude, which makes her almost impossible to hit anyway. She also has a 120% accuracy boost, so hand her some accuracy boosting glasses and a gun and she can kill just about anything. Honestly, the girl can make it all the way to chapter 9 all by herself. It's slightly balanced in that she's only available during a New Game+ in the DS version (She joins your party even on a first playthrough in the PC version), but it's possible to trigger a Non-Standard Game Over at the end of the first episode by losing to Mid-Boss, effectively gaining a powerful character while only losing about four battles worth of progress. And the New Game+ lets you keep Laharl, Etna, and any created characters at their original levels, so even that isn't a problem.
The Majin class. The Majin is quite powerful, having 110% proficiency in all stats for the first three ranks (meaning that all stat bonuses from equipment are 10% more efficient), which is an accolade normally reserved for specialist classes in their specialist stat, and 120% efficiency for the later three ranks (which is just flat-out crazy). This effectively means that they out-perform EVERYONE, at EVERYTHING. They're better swordsmen than Cosmic Heroes, better with their fists than a top-rank Eternal Fist, and better spellcasters than a Galaxy Mage(and have MUCH better health and defenses too). In fact, the only thing they aren't completely superior at is guns. At guns, they TIE with Space Marines, which are the highest rank of a class designed to use guns almost exclusively. The final rank, the Divine Majin, is flat-out superior and obsoletes all other classes once unlocked. However, this is somewhat counterbalanced by the esoteric requirements needed to unlock the class (At least one Brawler, Warrior, Ninja, Strider, and Thief all at level 200). Nippon Ichi also realized how devastating they were, so over the years, they have nerfed the Majin by completely dropping its MOV to 3 and their stats, aptitudes, and evilities have changed over the years to counterbalance its strengths.
Need something like a Majin early in the game? Then the Samurai class is for you. Unlocking it is very easy (Lv 10 Female Brawler and Valkyrie) and as soon as you unlock her, 110% ATK aptitude and S Rank in swords, axes, and spears! There are other classes early on that have 110% aptitudes, but having a great close ranger fighter is a must and the only other class that has those ATK aptitudes is the Majin class. Add in the fact that she has an S rank with swords right out of the gate and the general versatility that sword skills have, and you have a class that can last all the way until unlocking the Majin class.
In the original PS2 version it was possible to capture a level 9999 monster once you reach the fourth chapter by exploiting Enemy Level Up Geo Panels. This was sadly removed in the PSP version. Said high-level monsters could be used in a variety of ways -clearing Dark Sun stages with ease, allowing your weaker units to reap the rewards of massive EXP and equipment, unlocking high-tier bonus levels leading to even more rewards (Cave of Ordeals, The EDF missions), bulldozing through the item world, stealing, and if you wanted to, you could reincarnate them into a humanoid unit, and have a massive amount of stored levels and bonus points for allocating stats right out of the box.
The PSP remake of Disgaea 2 has seven bonus characters available as downloadable content, two of which are free downloads. Both of these characters join your party at level 100, one of the two (Pleinair) joins your party without a fight (and the other can be beaten once you have Pleinair), and both are available at any point in the game, including the very beginning. How gamebreaking is this? The final boss of the main storyline is only level 90.
This only gets more game breaking if you download other DLC characters as well, as every one of them is at level 100 and uses no equipment until you recruit them. Why is this game breaking? Because now that you have two DLC characters, you can use them to gang up on new character, who's only real threat to you is through their spells. If you rinse and repeat this with every character, then you can have 6 level 100 characters, one of which is purposefully devastating (Demon Lord/Majin Hanako). Consider your first cycle with these characters broken in half.
With a little reloading on facing down these DLC characters on a first cycle, you can also generate a Disc-One Nuke through the bonus gauge, as it's set to rank 20. This can get you equipment you can't yet afford, some extra money, or a small boatload of EXP. Done right, you can easily have your Level one characters bordering 30 by the time all the DLC characters are recruited.
The Ninja can be unlocked fairly early (Have 5 ranks in Sword and Fist Mastery, which Adell and your starting Male Fighter unit can get easily). Normally, you would never bother to use the Weight accessory (which reduces a unit to 1/4 HP) but on a Ninja, it's near-godly. When in critical health, the Ninja's special ability activates - his SPD (evasion) stat spikes to incredible levels. Give him a Weight, Shoes, and a Fist weapon and he'll be virtually impossible to hit, even with healing spells.
Clever manipulation of the Dark Assembly can net you some very valuable items and perks with almost no cost to yourself. There are six parties, three of which are very easy to bribe (Orcs, Wood Golems, and Mothmen) with any outdated/junk items you aren't using (Rank, rarity, and value doesn't really affect how much senators like them) to the point they will adore you. Once Adell becomes a Senator, he can take bribes (often items that aren't available at the shop until you reach higher ranks) and quit the proposal without losing Mana, and increase the influence of the parties that like him.
Hospital exploitation returns from the first game, and with a few hours of work, you can have a massive amount of money and powerful accessories at your disposal. You can do this in the very first chapter of the game, as your healing costs in the first chapter will be the cheapest.
In the third game, Adell is an extremely formidable DLC character. Evility that does 30% more damage in one on one combat plus a S ranked innate attack plus loads of enemies (including story mode boss fights) that are weak against fire equals pain.
Also introduced in this game were Fun weapons, each with a fixed innocent that had varying effects (such as restoring 50% HP/turn, gaining +5 counter attacks, even increasing ally monster stats). However, the one best known is the Puppy Paw Stick, an Axe-class weapon with the Cat Burglar innocent. Said innocent had a 20% chance to steal an item off any unit it killed. When combined with Clone panels or Dropouts from the Class World, it basically meant the ability to clone powerful pieces of equipment. NIS however nerfed this twofold in a patch: Clone panels can't affect player-controlled units, and the chance for the weapon to steal an item was dropped to 10%. This weapon also showed up in Disgaea 4, and is still as gamebreaking, though the RNG needs some manipulating to make it more effective.
The Puppy Paw Stick is in D2, though it has finally been nerfed to prevent item duping - it explicitly does not work on senators, and with no Class/Chara world, that's the only time you'll ever fight your own units.
To a lesser extent, Toy Hammer. Its innocent halves the HP of the target if you hit it with a normal attack from behind. When combined with the Dark History, which has an innocent that deathblows units under 25% HP, and a little luck, you can easily kill off enemies far stronger than your characters.
The Cheerleader class in Disgaea 3 has one skill that makes the game a joke. "Let's Go, Cha Cha Cha" What does this do? It allows a unit that has acted once to go again. Yes, that's right, take another turn within your turn.
And now the Vita version of Disgaea 3 introduces Stella Grossular, who's even worse than Adell. Her first Evility allows her to reduce the stats of anyone she's next to on the map by 25%, and if that wasn't enough, her stats INCREASE by 25% for each character next to her on the map. Her second evility that she can learn reduces resistance to fire attacks for all characters on the map by 30%. To top it off, her final skill is a 3x3 attack that is, wait for it, fire based. This makes power leveling her so easy it should be illegal.
If that wasn't enough, the Vita version also has Rutile, a half breed catgirl in the game. Stat wise, she's pretty good, but what makes her really good is that she's the best healer in the game, bar none, due to access to Tera Heal naturally.
Also added are Fuka and Desco, Desco has two evilities that are incredibly stupid. The first one makes it so that she can't be affected by non-special attacks. Put her in an area where everything is affected by forget or silence and she can't be killed by anything. Her final evility, at the end of the turn, paralyzes any enemy within 3 squares of her.
Factory Desco, similar to Mass Produced Kurtis in the first Disgaea game, has two evilities that make the post game an utter joke. Her first evility allows her to recover 10% HP and SP per enemy unit she defeats in battle. Considering that all of her skills are attacks that can hit at, bare minimum, four enemies at a time, she can go from nearly dead to back to full health in an instant. Her final skill, learned only at tier 6 and level 800, allows her to magichange with anyone. In Disgaea 3, you need to put a monster in the same "club" as a human to magichange with them. This evility bypasses that completely and allows for some really awesome tricks to happen in battle.
Some consider Desco of Disgaea 4 to be a game breaker due to her powerful evilities (Increases attack of adjacent allies by 20%, gains 20% attack per third of total HP missing, and deals 50% more damage at the expense of receiving 30% of normal mana), high stats, and multitude of long ranged area attacks that become even better after a monster fuses with her (Yog Sothoth hits a 3X5 area up to six spaces away, making it the biggest ranged area attack in the entire series). On top of that, she can be boosted even further by a Beast Trainer.
She's also exceedingly easy to Level Grind once you get to Chapter 9. Stage 9-4 is tailor made for Giant Desco to kill 15 enemies in a single blow, with a 50% boost to both Mana and XP. Most of the really good grinding tactics (Mt. Ordeal 4 + Big Bang, for example) are either not available until the post-game, or require pretty powerful characters to begin with. Not this one, with almost no effort you can easily have Desco (and all your other monsters, since someone must fuse with her) solo the entire rest of the story.
And now we have the game breaker to end all game breakers: Tyrant Valvatorez. His lowest item proficiency is 110% (the rest ranging from 130-150%). He automatically increases the stats of whatever weapon he wields by 50%. His stats can be boosted by 80% simply by having four adjacent allies at the home base. And then there's his evility No Life King, which revives any enemy he kills as an ally. The story was not kidding when it said that Valvatorez was once absurdly powerful.
King Krichevskoy is also on the same league as Tyrant Valvatorez. Not only his item proficiencies are on par with the Tyrant, as he can inherit his Prideful Comrade evility (80% increase in all stats if he has 4 units around him in the home base) and his evility is as reliable, if not more than Tyrant Val. Remember the 30% damage boost on one-on-one skills that made Adell the most powerful character of Disgaea 3? Krichevskoy's main evility gives a 50% damage boost on ALL of his skills.
Overlord Zetta gives both Valvatorez and Krichevskoy a run for their money with his evilities. His second and third evilities give him a 30% stat boost to either atk, def, int, or res, depending on which one's equipped and if it's an odd or even turn. That doesn't seem like much until you take into account his first evility, "Badass Overlord", which allows him to use any special attack in the game to be powered by his strongest stat. To put it more simply, if you build him as a sword fighter, you can teach him Tera Star and instead of it being powered from his int, it's powered by the strongest stat (which would be attack).
In another note, on Disgaea 4 it is possible to pass on the Trick Move evility to any and all classes and characters in the game - in the previous game, it was restricted to the Joke Character Masked Hero class (which kinda made it a Lethal Joke Character of sorts). Trick Move gives you a 10% chance of evading enemies' attacks for each panel your character moved on that turn. The thing is, once you have a decent favor rate of the congress, you can upgrade any legendary item in the game to make it increase the move of the wearer by +6 on top of whatever benefits it already had (such as the Accelerator, with a natural +2 move). It is possible then to make the tank classes to be able to move more than 10 panels in a single turn. With Trick Move, when the enemy turn comes your character will have 100% evasion, meaning no one will be able to hit him/her, even if they are 1000 levels higher and at max accuracy.
The presence of the cheat shop makes it incredibly easy to level up characters, and the further you get into the game, the easier it becomes. It's possible to level up an entire party at a level called the Inner Sanctum (post-boss battle), by forming a ten character tall tower with a character with defence high enough to ignore the enemies there at the bottom. The enemies will automatically approach anything on the field as soon as something is deployed, so you don't have to move. By setting EXP and Mana gain to max and minimized HL gains, you can level up your characters very easily, so long as you keep the enemies at a level that's 8-10 levels below that of the character at the bottom. At chapter six, you can end up with a Laharl that's 200+ in a few hours, making the rest of the main story a cakewalk.
The cheat shop also affects HL and Mana gain as well, literally giving you Money for Nothing and more then enough Mana for those bills. Not to mention Rasetsu Mode, which raises the item level cap to 999.
Jack monsters have an incredibly powerful evility available. This evility reverses the effects of status effects, which are more common in this game than in previous titles. What this evility means is that, if inflicted with poison, they regenerate health. If inflicted with blindness, they become MORE accurate. If inflicted with deprave, they gain major boosts to all stats. Given that status effects like poison can still potentially kill even a 9999 level character if inflicted by a level 1 character and not dispelled (since poison works off of a percentage instead of a flat rate, and takes 20% of a character's health per turn) the character that can GAIN power from those effects effectively becomes a powerhouse. Teach one of these creatures spells that let them inflict status effects, and you can have them boost each other as they tear their way through your enemies.
DLC character Salvatore from Disgaea 3: Absence of Justice has multiple evilities, all of which are game breakers and affect all adjacent allies; increasing their movement by 10, the range of attacks and spells by 9, jump by 99, attack by 100%, and reducing all damage by 90%. The only caveat is that the evility is random and changes every round of combat. But whichever you get, it's probably powerful enough that there's no reason not to field her, and you could always reload a battle until you get the one you want.
Made worse by three factors: one, Salvatore retains whatever effect she had active as long as she is sent back to the base panel at the end of the turn, meaning that the damage and attack range boosts can be used every turn. Two, the evility also affects monsters she's mounted on, meaning you can hand out five massive boosts instead of four while also protecting Salvatore in the first place, which works ludicrously well with the defence boost. Lastly, you can transfer the evility to other characters, which, if you decide to go nuts with it, can grant all party members' numerous, massive buffs.
Similarly to Salvatore, Pram, another DLC character has the evility Prophecy, which also has a randomized effect per turn. The effects include attacking a random enemy, doubling the user's attack power, increasing the INT stat of all allies by 50% and inflicting poison to all enemies. And you can use Save Scumming on non item world maps to get the effects you want.
Marona. In Phantom Brave (Where every unit but her are ghosts by default), her "Confine" power was a simple mechanic. In this game (where the units are alive and the Base panel serves her role as the starting point) She can bring back units that were defeated in battle with a 30% stat boost. Heavy Knight took the bullet? Back and able to take more hits. Glass Cannon or Squishy Wizard dies? more resilient and can now deal more damage. Fragile Speedster (Especially Masked Heroes with Heart of a Flea) got trampled? faster and allows him to carry your through the item world from beyond the grave. The only downsides are that Marona herself is a Squishy Wizard, she cannot act the turn she confines a unit and that her Confine is a spell that starts at a range of 1. (But that can be mitigated with Range boosting effects like Petta's or Salvatore's evilities.)
Human Priere comes with the evility Purify Power, which, upon inflicting damage on an enemy, causes the damage to radiate out to all adjacent enemies, getting weaker the further it goes. With the right set-up, this has the potential to clear an entire map in one attack. What makes this especially powerful is that user gets all potential gains (EXP, Mana, HL, and Land of Carnage stats) from all enemies defeated this way. This is a must for getting the max stats through the Land of Carnage.
What also makes Purify Power amazing is that it can be used as part of a larger strategy to effortlessly clear Rasetsu + Land of Carnage Item World Floors with impunity, maximizing stat gains on your equipment.
The return of weapon mastery brought with it monster mastery, which boosts the stats monsters gain when they equip monster weapons, which increases based on level. One can already spot the problem with this - levels are easier to increase than weapon mastery, on top of caster monster weapons increasing spell range and damage dramatically. Simply having an army of semi-high level monsters is enough to rip the game to pieces, including some of the more complicated bosses, like Xenolith and Darkdeath Evilman.
Valvatorez has his Absorption Evility increased to gain 20% of a slain foe's stats. What makes this game-breaking is that unlike other stat-increasing Evilities, such as Laharl's Overlord's Dignity / Pride, is that Absorption does not cap. With the Rasetsu modifier on, this means Valvatorez can hit the stat cap with ease, making him nearly perfect for taking down Tyrant Overlord Baal.
Christo comes into the fray packing an extremely powerful Evility that gives any adjacent ally perfect accuracy and 25% evasion, a potent heal that targets all allies in his general vicinity, and one of the strongest Overloads in the game (see below for that), all as soon as you get him in Chapter 2. As the story goes on, he obtains access to a solid 3x3 ranged combo skill with Usalia, another Evility that grants adjacent allies bonus ATK equal to 30% of his INT, and an upgrade of his already ridiculous Overload. Most importantly, you never have to worry about grinding for him. Christo's Evilities and Overload are so powerful that he can get away with being level 50 or so for the entire game.
Usalia is notoriously hard to take down, especially when used in tandem with Christo. Optimal builds result in a minimum 145% damage reduction and a max of 270%. Considering that offense got nerfed and armor finally got proficiency, this toughness is even more exploitable. All on top of 75% evade rate. Her Overload "Murmur of Rage" is considered one of the best in the game, being one of the best ways to take down Baal.
One of her common evilities, "I Love Curry!" has a incomplete description. It says that after eating any Curry item, the unit will also deal 30% more damage for three turns. In addition to that, it also makes the user completely invincible for three turns, as in, even immune to Baal's attacks. The only way a character can die while under the effect is to kill a Baal Node (wich has an evility that causes the unit that killed it to be killed in return). And since you can get an infinite amount of Curry items with the Alchemy Squad...
Maids make for amazingSupport Party Members. Their Unique Evility, Efficient Work, lets them use one item without having it count against their "one action per turn" limit, meaning that they can move someplace, use an item, and then move again and take an action. One of their Evilities, Know How To Use It, doubles the effectiveness of healing items; you eventually unlock an item that refills 50% HP and SP, Curry Soup, and that Evility turns that item into a full heal, and Curry Soups can be mass-produced at the Alchemist in the post-game. Another one of their Evilities, Licensed Caregiver, expands the Area of Effect of healing items into a five-panel cross shape, allowing the Maid to fully heal or treat status effects for up to five units without any of them using their turn up. And finally there's their Signature Move, Afternoon Tea, which grants an Extra Turn to another unit. Every other active skill on this list that isn't an Overload can be used again.
The Sage has been accused of being significantly better than just about everybody else, and it sticks out like a sore thumb given how surprisingly balanced the generic classes were this time around. Their innate evility Mass Blaster grants them +10% extra damage for every unit they target, which works not only with spells but with any area of effect attack in the game. Their first unique skill is Multi-Attacker, which allows them to give an ally the ability to attack twice in a row for two turns, effectively doubling their damage. Worst of all is Land Decimator, the Sage's second unique skill and quite possibly the best attack in the game. It deals a very low amount of damage and only has a 50% hit rate, but it also targets every single unit on the map, which when combined with Mass Blaster makes the poor damage moot. She's no slouch against single targets either, as she can learn the Concentration Evility, which doubles the damage of single-target magic attacks. A sufficiently geared Sage can solo the entire post-game by herself, up to and including the Item World.
Among the DLC we have Artina, Fuka, and Desco working in tandem. Fuka has a combo skill she can perform with the others called "Tri Angels" that attempts a Steal maneuver against every enemy on the map. While you don't get to pick what you get, and its effectiveness is capped at 50%, you can try it over and over again until you get what you want. Absurdly useful in the deeper levels of the Item World, and if you are ever strapped for cash, unleash it in the map featuring 100 Asagis to watch hundreds of millions of HL worth of gear fill your pockets.
Valvatorez is also among the best DLC characters. His unique Evility Absorbtion is fantastic (absorb 5% of the stats of every unit he defeats, meaning he gets more powerful the longer he fights), his Overload which turns him into tyrant Valvatorez is likewise great (any enemy he defeats gets resurrected as an ally), but his *real* strength is in the Evility "Prinny Instructor". It increases the XP, Mana, and Class XP gain of any Prinny in the same Squad as him by 100%. Good by itself, amazing once you pass it out to other characters and then put them all in the Prinny squad, which makes all members count as Prinnies. Prinny Squad has a maximum of 10 members, so now all of them can get 1100% XP, even without the benefit of Statisticians or the Cheat Shop. Combine with Fuka's "Prinny Hat" Evility, which makes any character count as a Prinny regardless of Squad, and now you can do this trick with one of the Squads that allows 20 members, bumping the gain to 2100%.
Overloads are powerful, once-a-map abilities that can only be triggered in Revenge Mode. Some of them are simple stat boosts, but others are just plain brutal:
Christo's Occhi Clairvoyance grants a 30% boost to accuracy and evasion to all of his allies for one turn, temporarily making your team harder to kill while making the enemies even weaker. Even better, it's one of the first ones you get. The upgraded Evangel of Purity upgrades the bonuses of to 100%, making your team borderline invincible for one turn, unless you're fighting Baal.
Usalia's aforementioned Murmur of Rage is hilariously loaded. Just by triggering it, she deals damage to every enemy on the map, and transforms into her Berserk state. In this form, she auto-attacks any enemy who is hit, anywhere on the map a limitless amount of times until the form expires, and she swaps her old skillset for a 3x3, high damage skill.
Killia's Tyrant Revelio simply lets him take 3 actions in one turn. Considering he has access to a one-of-a-kind skill that deals monstrous damage to one target, a sufficient amount of boosts can allow him to destroy almost any boss.
Etna's Chaotic Ordina summons an army of Prinnies to fight for her. And we mean a lot of Prinnies. While their stats aren't the best, they are very effective meat shields for the rest of your team, and they can build up the combo meter to astronomical proportions, especially in conjunction with Usalia's Overload.
Izuna's Comet Disaster is a simple Star-elemental attack to all enemies. In the postgame, put this on the aforementioned Sage class and it becomes Land Decimator but without the accuracy penalty or "target friendlies too" property.
Majorita's Broken Faith Magia, an Overload so strong that the game won't even let you use it until you unlock Void Dark and pass a bill to let Majorita have it back. This monstrous ability re-summons every single dead enemy under your control. For example, if you're fighting an Item God but you managed to clear all of the trash mobs in the room beforehand, you can summon them all to swarm the boss and deal a good amount of damage in the process. The best use of it is against Baal, where once you kill his avatars, you can summon them back to deal the finishing blows, with no hope for him to retaliate.
Petta's Future Eyre gives all allies +1 Overload use. No exceptions. Every Overload on this list can be used again.
Certain Evilities can also massively tilt things in your favor:
The Twin-Headed Dragon's Twin Head Evility allows for a second attack as long as the unit hasn't moved since the beginning of their turn. You can immediately attack twice, and then move. This can also combine with the Nekomata's Final Blow Evility — if the enemy survives with around 10% HP or less, you attack again!
Emizel's Death March grants a 2% damage boost per enemy defeated. Majorita synergies well with this, as her Queen of Coffins grants her a 10% attack boost per enemy beaten, and it can also stack with the Horseman's Combat High, which does the same thing. On a large map with plenty of foes, you can hit immensely hard.
The Kunoichi has Nirrti, which grants a 10% stat boost per attack dodged. Not only can Forced Counter and Counter Dodge max out Nirrti's boost, but it also triggers on passive HP/SP restoration, such as the Zombie's Corpse Cannibal (3% HP/SP restored per enemy killed), the Fairy's Magic Generation (10% SP restored per turn), and the Spear Fisherman's ability (restore 3% HP per attack landed).