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 then 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.
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 then intended, and break the game with the absurdly powerful equipment and tremendous EXP rewards from them.
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+, 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 hideously overpowered, having 110% proficiency in all classes for the first three ranks (meaning that all stat bonuses are 10% more efficient when equipped on them), 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 simply flat-out broken 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 at lv 200). Nippon Ichi also realized how broken 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 purposfully super strong (Demon Lord/Maijin 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 broken 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 Burgular 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 Disgaea4, 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 bi-passes 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 broken, 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 Disgaea4 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 (wich 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 whathever 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 defense 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 witch 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 defense 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 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.)
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.