Just because Shin Megami Tensei is infamous for being Atlus Hard doesn't mean you can't break it. Hell, in some cases, these breakers just might be your only chance at victory.
Fusion in general can get you some hilarious results in either Persona 3 or 4. Skill inheritance can get you a persona with all 3 full-party autobuff skills by mid-game. Most encounters won't survive 3 turns of physical attacks if a player uses debuffs as well.
Pretty much every game features a fusion system that can lead to game breakers, but Devil Survivor takes it a step further by allowing you to actually choose exactly what you want your demons to inherit. The only skills that you can't control inheritance of are Race skills, but everything else available on the component demons is fair game.
Most games have Victory Cry (sometimes with another name, such as Auto Soma), a passive skill that completely heals the character who has it after every battle.
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Shin Megami Tensei
In Shin Megami Tensei I the Zio line of skills stun almost everything in the game. With high enough speed, the heroine can immobilize pretty much everything before it even has a chance to move. The Bufu line of spells works just as well, but you need demons for those.
Marin Karin works on the vast majority of bosses. Get it to stick, and you can sit back and watch the boss beat itself to death.
Also in Shin Megami Tensei I, one of the best swords in the game, the Hinokagutsuchi, can be acquired about halfway through if you know the proper fusions. One attack is usually enough to kill anything short of the final boss and his lackeys.
In Shin Megami Tensei II, it's damage and defense buffs and debuffs, which stack with themselves and last until they are dispelled, something that not every boss is capable of. But more-so is the Divine Retribution spell. It tears off a quarter of of an enemy's health with an alignment opposite to to the spell's caster, and it works on bosses with no reduction.
Speaking of stackable stat buffs, there is a passive Game Breaker that can, if one is not averse to Level Grinding and item farming, turn your main character into a stat buffed tank. This is achieved by farming gemstones from demons until you have nine of each type, whereupon stats buffs are applied to your permanent stats as long as you don't reduce the gemstone count, and this condition is stupidly easy to honor, since the only two ways to lose gemstones is to spend them as currency at a certain store (which can make the game easier for some gamers but never need be visited) or use them as trade items in demon negotiation, which, again, need not be done because all demons can be bribed with other materials. This doesn't mean that the stat buffs make passing the skill check for the bonus dungeon easier (the check goes by your natural skill level, ignoring buffs), but this can be used to make certain story event skill checks effortless.
In Shin Megami Tensei If, having save data from other Atlus games when you reach the Appraisal Shop in the World of Sloth will get you 18 extra stat points (effectively letting you reach level 117 in a game with a 99 max), increases all of your demons' HP permanently by 25%, gets all of the buff spells on your partner, Sabbatma, and ten somas. That's ten party-wide full-HP and MP healing items, a spell that lets your partner summon demons, and the three most useful spells in the game!
Shin Megami Tensei III: Nocturne has its fair share of Breakers too, in many forms. Masakados, for example, nullifies just about every attack on you and increases strength, vitality, magic and agility by 10 (the max on stats in this game is 40). Its abilities are also ridiculously powerful; first is Megidolaon, then a very powerful Expel-type move, and then the ability to reflect all elements and physical moves.
Most of these abilities, though, aren't as powerful as they might seem —- first of all, by the time you can acquire Megidolaon, you are probably already at a fairly high level, reducing its damage considerably (magic damage, unlike physical, decreases with levels). Second of all, the Expel-type move only deals damage, which has very few, if any, practical uses. Finally, physical reflection is pretty much the only useful reflect skill of all, as enemies who already use spells are extremely likely to be at least resistant to (or, at that point, actually absorb) the element they cast.
Then there's Fog Breath, which drops the accuracy and evasion of every enemy by two stages and can't be blocked. You can get this skill from a common Magatama as early as Level 21, and although the MP cost is steep at that level, Fog Breath retains its value right up until the endgame.
God's Bow and Hell Gaze are basically the same attack, just with different elements (Bow is Expel, Gaze is Death). Either they can block that element, or they die. Simple as that.
Beelzebub (Fly form) has Death Flies, a skill that has 100% of killing all enemies unless they are immune to Death. If they are immune to Death, they take an amount of damage equivalent to Megidolaon.
Hoo boy, Black Frost. Where to start? He's one of few demons that can be recruited outside of fusion, he has ridiculously high stats, comes with a Ma-Dyne spell, and has no weakness. And his resistances? He absorbs ice, reflects fire and darkness, is immune to light, and takes negligible damage from physical attacks. This means the only attacks effective against him are electricity and force.
Even though he joins the party outside of fusion, he can still be fused and/or have Mitamas fused to it to make him even stronger.
Atropos. When you get her she has Element Boosts, which amp up damage of a specific element by 50%. In other words, she is optimal for being a Quad-dyne-spell user.
Daisoujou. Immune to every status ailment in the book in some way, has good Magic stat, and can restore its own HP and MP during boss fights with Meditation, and also learns Prayer, which makes him a stellar support caster, especially if you sacrifice demons to level him up.
Recarmdra, with proper exploiting and fusion, you can create an infinite cycle of healing or one powerful mage. When the HP/MP of your party is low, have that demon use Recarmdra, Blammo; your party gets back all their HP and MP, next have another demon use Samrecarm to resurrect it, and summon it again. Mana Drain also is useful, give it to a powerful magic user and let them do all the damage dealing; it is even better if they know Megido class magic.
Freikugel: Almighty damage, stupidly high critical chance, absurdly high damage output. It's simply the strongest damage skill in the game. In addition, on the True Demon Ending, due to it being an odd mix of Physical and Almighty, you can combine it with Pierce to break one of the last bosses in the game, who is a Barrier Change Boss resistant to Almighty.
Focus: Raises the damage of the next physical attack by around +150%, and can be learned as early as lv23. It's not hard at all to hit 4 digits as early as against Matador (who only has 1000 HP), or 5 digits later in the game due to this.
Shin Megami Tensei: Strange Journey has Victory Cry + Jihad (or any powerful Almighty-type spell) on a demon. The Eternal Rest spell in conjunction with any sleep spell is also effective, as status ailments work quite often on a surprising amount of enemies, and the combo can lead to a near Disc One Nuke if you get it early enough.
Shiva as well as The Demiurge appear as the demons with the highest starting level for the Chaos and Law alignments respectively. Both naturally start with the aforementioned Victory Cry + Jihad combo and have respectable resistances (both are immune to expel and curse and have several other elements which are ineffective against them). Additionally Shiva has Enduring Soul (a full revive after death once per battle) and The Demiurge has luster candy (boosts all stats for the party). Shiva gets bonus points here for being considerably easier to summon (he only requires the player to be level 91 and is created in special fusion with his traditional recipe, while The Demiurge is the bonus boss and must be beaten, only in the second playthrough and later, before you can fuse it from 3 specific demons.)
There are several guns that allow you to cause the Stone element. This counts as instant death. Almost everything except bosses are weak against this attack. Your Main Character gains 4x the EXP he would normally get if you have no demons out. Manshonyaga (an early game weapon with the stone element) will allow you to gain levels every 2-3 fights for a while.
The 4x EXP bonus can also be gotten by softening up an opponent with your demons then giving them the return order and finishing the battle on your own. This makes it trivial to break the game in a different way.
While Jihad is commonly considered the ultimate skill, it isn't necessarily the most damaging. That falls to the ultra-rare skill Desperate Hit (only naturally on Demonee-ho, and can only be passed on via demon source), which does a random number of almighty hits. If it does 3 or more to a single enemy, that enemy takes more damage than it would from Jihad. VERY useful agaist bosses. Not to mention it's only 40 mp.
The way Strange Journey's reflection equation works (damage is calculated entirely based on how much it would have done to the target of the attack, not how much it would do to whoever it's reflected back onto) a smart enough player could have a team of low level demons reflecting attacks back for THOUSANDS of damage, only having to heal their Main Character occasionally and watch out for Almighty moves.
Shin Megami Tensei IMAGINE has Erosion Hex, a skill that can hit paralyze, poison, amnesia and stone all at the same time, and has a huge area of effect. When combined with the soulstones of Mothman (+50% Amnesia rate) and Catoblepas (+50% Stone rate), this skill can make everything not immune to mystic/amnesia and death/stone easy targets. It has such low requirements to be learned that nearly every build in the game can incorporate it with minimal effort.
Magic Bullets have the Enchanting Bullet Hole, which has a base 40% chance of hitting charm. Then there's the nekomata soulstone that raises that chance by 50%. And then there are dozens of different ways to get the 10% remaining to have 100% charm. Considering Magic Bullet skills completely ignore enemy resistances for calculating ailment chances, anything that doesn't null mind/charm will become charmed when shot by this.
Shin Megami Tensei IV brings back Demonee-Ho's Desperate Hit... which is now more useful considering that it can be passed onto your character. Basically, it's an Almighty attack that hits enemies multiple times, but it also has physical attack attributes. Not only does it bypass all weaknesses, but it can also be charged and can get critical hits. Combine with the MP recovery apps and you'll find it hard to use any other skill.
The MP recovery app is broken in itself. You know it's broken when you can use Megidolaon every battle and have no shortage of MP by the next battle.
Remember Jihad from Strange Journey? It's back in this game as Antichthon.
The Skill Augment app. It makes demons more likely to mutate skills. With a little luck, it's possible to get useful skills early in the game and get even more powerful skills as you proceed.
Odin's Thunder Reign attack is a Severe-level, all-targeting Thunder skill...that costs only 20 MP. To compare, Maziodyne is one tier lower and costs 32 MP. While Odin himself has a pretty bad Magic stat, Thunder Reign can be passed on either via Demon Whisper if you have a high Magic stat, or to a new, magic-oriented demon via fusion.
Lucifer came very close; he had 999 or 99 in all stats, some of the strongest Dark skills, was immune to everything, and had the strongest earth skill. However, he was completely useless against dark demons as he had no light skills, so anything that reflected light would kill his host (always Reiji/Chris). note Adding a light skill through Tablets would circumvent this problem, however. Instead, the real Game Breaker was Abracab; it had the strongest light skills, the strongest healing skill, and the generally brilliant Boss Damage. That's not the best part, however; the real fun was Bi Strike, which did damage equal to twice the user's HP. If your level is high enough to summon Abracab, your HP will be so high that nothing will be able to stand up to Bi Strike. But you need revive magic, of course, which costs a lot of MP... actually, that's not an issue for two reasons. First, MP cost is calculated by the Persona, not the skill, and skills could be added to a Persona that didn't learn them naturally, meaning you could abuse the system and, in theory, get a Persona with a single-digit MP cost that had Revive More. Second, dead characters are revived with 1HP after the battle ends, meaning that all that's needed is healing and, if you're able to summon Abracab, you've probably found enough Spell Cards and got a high enough level that you have something with Elixir All (possibly for a low MP cost, as previously stated) By the way, Abracab himself has Elixir All, and since the dead character will be revived, he or she can then use Elixir All on him or herself, making Abracab 100% ridiculous.
In , Lucifer and Satan had a spell, "Alpha & Omega" which skipped the damage numbers and just wipes out the enemy party. Only two bosses in the game actually had some sort of countermove for it. One is an enemy that is immune to all attacks until you remove its barrier, and the other is Philemon, who will reflect the attack on YOU and cause a Total Party Kill.
Before this are the spells Hell Desert, Megalo-Fire, Storm Nightmare and Maelstrom. It's technically a Total Party Kill that can only be avoided by one of two means; luck (as these spells are ungodly accurate) or resistance to a certain element (in respective order; Earth, fire, water or wind). These spells are available even earlier than Trimurti, however.
What is game breaking is Eikichi's ultimate weapon, the Supernova, in Persona 2: Innocent Sin. Not only is it the most powerful weapon in the game, but you can also get it very very early. It allows you to mow through the first half of the game and some of the latter half with extreme ease. Downside is, it's an extremely difficult weapon to get.
One broken spell combo is "Trimurti" by the personas Shiva, Vishnu, and Brahma. It will KO all non-boss enemies. It is more broken since these three can be obtain far earlier than Lucifer and Satan and do not require the insane MP consumptions. This only applies to Innocent Sin since it was nerfed in Eternal Punishment.
Pegasus Combo might not be that Game Breaker, but being able to deal about 400 damage points since level 10 is something to consider.
The Lucifer (or Helel in the remakes) persona (in addition to being the most powerful persona on its own, bar none) gives access to "Armageddon", a spell that deals 9999 almighty damage (cannot be blocked or absorbed) to all enemies at 100% accuracy. While it will kill every non-Bonus Boss enemy in the game, Armageddon in itself isn't a Game Breaker, because it costs 100% of your MP to cast. No, what makes it a Game Breaker is that another of Lucifer's abilities, "Victory Cry", replenishes all your HP and MP at the end of each combat. End result: You win everything.
To add insult to injury, it's fairly easy to give Lucifer the "spell master" ability, which halves all MP usage. Yes, that includes Armageddon. Now, even going into combat in less than 100% shape lets you win everything as well.
It was nerfed in the sequel, a little—Lucifer (now Helel) no longer learns Victory Cry naturally, and one might argue that it can be a pain to get it on the fusion components.
What!? Lucifer is still in the game, part of the Judgment arcana now though. And he IS a fusion component for Helel, meaning it is absurdly easy to get Victory Cry on Helel, thus becoming an unstoppable snowball once again.
Outright removed in Portable, where Fusion Raids are done via items that require grinding for items (and Armageddon requires 99 of the main item and 10 of a hard to get one).
However, Portable brought a new Game Breaker to the table: Skill Cards. Think TM's from Pokémon, only 1) you can easily farm them and 2) there are no restrictions to what persona can learn what skill from cards. This means you can get personas with skills that they couldn't have through fusion (ex a Fire type casting Bufu spells). While it doesn't have the unique, superpowerful skills (Morning Star, Black Viper, Die For Me, Thunder Reign, Ragnarok, stuff like that), it does have the "Absorb x attribute" skills. This makes it trivially easy to cover up weaknesses in strong personae. This can be taken to Nigh Invulnerable levels with a Beelzebub that essentially Blocks, Absorbs or Repels every attribute (and still have slots for attacks!).
Also of note: Skill Cards can be reproduced for no cost. The Inari Sushi in the temple can take one card and make a copy of it after a few days. If you religiously go there every time he finishes, you can load up on very powerful skills you can put on any Persona you want.
Portable also introduces Direct Commands. This in itself would not qualify as a Game Breaker, but consider that the PS2 versions do not let you directly control your allies and that P3 was designed with AI-controlled allies in mind. This is especially useful on bosses such as Chariot and Justice, who must be killed within the same turn or the one that is killed gets fully revived, as well as the Final Boss, who has a skill that repels all attacks; in the PS2 versions your allies will idiotically attack the boss, but here you can make them, you know, not attack until the repelling skill comes off.
Also broken is the new Great/Good/Tired/Sick system. Combined with the trivial cost of paying the clock for healing (and, later on, the large gobs of money you can get from higher-level Coin cards), you can easily travel from one temporal barrier to the next in a single night, as you and your allies' condition only change after you exit Tartarus. Yes, this will put your whole party into Sick status the next day, but that doesn't affect social activities, and at worst you'll be out of battle condition for a couple of days, given a day of going to bed early and investing in condition-restoring services and items.
In fact, let's just say that P3P is Game Breaker incarnate. It's practically the perfect game for SMT beginners, even excluding the two Easy modes.
FES Also brings the MC's ultimate persona: Orpheus Telos. This palette swap of your original persona is clearly specifically designed for the Bonus Boss because it resists everything but Almighty and Poison (The Bonus Boss will automatically kill you if your persona absorbs or repels any of her attacks, so resist is the best possible option) and has great stats. The only spell that it learns naturally is Victory Cry, but most importantly, it can inherit every skill in the game. If you plan everything out right, you can have a single persona with the eight best attacks in the game. Mix and match to your hearts content to decide what eight skill combination is right for you. The catch? He can inherit every skill in the game, not just the best ones, and because it has an equal likelihood to inherit every skill, you'll spend hours wading through combinations with inferior skills, hoping for that one-in-a-million perfect combination that you just spent a few hours setting up. Of course, this is all assuming that you maxed out every social link in the game, have the necessary fusion fodder, and are above level 90; the requirements for being able to fuse Orpheus Telos.
The Omnipotent Orb accessory makes you nullify every element other than Almighty, being this game's version of Nocturne's Masakados Magatama. There are only 2 ways of getting it, though: one is by defeating the Bonus Boss, and the other is a 1% chance at a Heart Item from Messiah. Of course, using this against the Bonus Bosswill get you killed, but it doesn't stop you from destroying everything else on your path.
Though spell combinations weren't included, Lucifer makes his return as the notoriously hard-to-fuse but gloriously broken ultimate Persona of the Judgement arcana. He learns both Victory Cry and Spell Master naturally and tears up Shadows like nobody's business. Even more broken (and even more of a pain to get) is the game's sole Persona of the World arcana, Izanagi-no-Okami. This one starts with Victory Cry as a base skill, and proceeds to learn 4 high-level elemental skills and an amping skill for each one. He is a bit Awesome, but Impractical, because he doesn't inherit any skills, and has no immunities nor any protection from Insta Kill Light and Dark spells.
Trumpeter, on the other hand, starts off Immune to Dark, Reflecting Light and Electricity, and Absorbing Ice. As a Hexagon fusion, it is moderately easy to get the other three immunities, giving you a person immune to everything except Almighty and Poison. On top of that, Trumpter gets the best buff and debuff spells and the strongest non-unique Almighty spell automatically.
What happened to Yoshitsune? Power Charged Hassou Tobi DOUBLES the damage output of a Power Charged Primal Force, the second-strongest physical attack in the game. Not to mention no weaknesses, and Yoshitsune is one of the most customizable Persona in the game (he only needs Power Charge and Hassou Tobi, and you can technically get Power Charge off of a slave), meaning you can have a semi-invincible god slaughtering the field.
Also, he has Heat Riser, which is essentially the combination of all three buff spells, and when fused on a certain date, he can also learn Debilitate, which is a combination of all three debuff spells. Combine it with the aforementioned Power Charge and Hassou Tobi and you will have a combo that could defeat Death with at least two usage of it.
Beelzebub in Persona 4 is the ultimate tank Persona. As long as you give it something to counter physical attacks, then the only thing it won't absorb, nullify, or reflect is wind and light.
Granted, it's possible to get Null Light and Repel Wind on him, thus making you immune to all damage that isn't Almighty.
Metatron can do the same thing as Beelzebub but is a little bit harder to pull off.
Random skill changes can give you Victory Cry at Level 24. No, really. Kaiwan in P4 comes with Tetrakarn, which randomly changes to any other equivalent skill on certain days. A lot of save scumming can end you up with Victory Cry at a ridiculous level.
Not to mention that this Victory Cry can then be spread to other Personas. It's possible therefore to carry a Victory Cry slave throughout the whole damn game.
That's not even scratching the surface of how game breaking this skill change abuse can be. You could get all of the Repel [Element] skills (including the ever useful Repel Phys), Brave Blade, Mediarahan, Spell/Arms Master, and a dozen other high level skills (all of which would otherwise only be available by the last third of the game) as early as April/May.
Norn is this if you're willing to invest in a little Save Scumming. Having Status Buffs on your entire party at the beginning of a fight is pretty good so that you can do a lot of damage right off the bat. If you can also get her Repel Elec and level her up till she gets Debilitate, you pretty much have a good starting Persona for boss fights (though you could still just use Trumpeter for the Debilitate part).
The level up trick with the pringles is also a good way to have a high level. While unfortunately, there's a cap for EXP, there's none for cash. Let it summon its mooks, turn on Rush Mode overnight and wake up with a lot of exp (at 65k normally, 130k with both equipments (each increasing your exp by 50%)) and a million worth of cash that you could use on a New Game+ (you know, to summon your broken personae listed above). Though as for the two equipments...
Unlike the previous game, you do not keep your level or stats. Since you have low SP, summoning late-game Personas with high level spells becomes impractical. However, physical based Personas are fair game, because physical attacks are percentage based, rather than having a fixed spell cost. If you have Arms Master, then severe physical attacks would now cost only around 18 HP; all physical attack Personas practically become Game Breakers for a third or half of the game! Bonus points if the Persona you're using is the above mentioned Yoshitsune. Oh yeah, and if the Persona nulls or even returns physical damage, it would make Shadow Kanji a complete joke.
In Golden to add some Anti-Frustration Features, you're allowed to choose what kind of skills your Persona inherits. All the aforementioned Game Breaker stuff listed up above? Oh boy...
Other Shin Megami Tensei
The Null Attack skill from Digital Devil Saga nulls everything except Almighty attacks. This makes you effectively invincible for 90% of the game as only the last two dungeons have random encounters that make use of almighty attacks and none of the storyline bosses (except for the last boss) use almighty as their main form of attack. It won't help you against the bonus bosses though since they just love their powerful unique almighty attacks and the Demi-Fiend will rip you to shreds on his first turn if you dare to have immunities equipped.
Cocytus in the first game. It's a random-hit ice spell that has an absurdly high rate of inflicting the Freeze status, which makes the target lose all their resistances and turns any physical attack on them into a Critical Hit. Also, if it hits a same target twice, it deals more damage than Bufudyne, the top tier ice spell. Slap Ice Boost and/or Ice Amp, and the game will be a walk in the park for the most part. It's especially useful against the Boss in Mook Clothing Arahabaki enemy, which repels everything but Ice (which it's weak to). It was nerfed in the second game, though.
The Death Flies skill from Nocturne makes a comeback in this game: 100% kill if targets don't null Death, Megidolaon worth of damage if they do.
In Devil Survivor, some skill combinations certainly count. Most notable, however, is Drain. It does Almighty damage (which means that 99.9% of enemies can't resist, null, deflect, or absorb it), is surprisingly damaging, and robs the enemy of their HP and MP while replenishing yours.
Also, Holy Dance and Drain Hit+Power Hit+Any Phys passive buff.
And also: Tyrant + Megidolaon + leader with Magic Yang = free one-turn kill every turn. And if you've got Awakening as well, even better!
In the Overclocked remake, this is expanded with the combination of Makajamon + Grimoire + Death Call. Makajamon has a high natural chance of success (higher than Petra Eyes) but just mutes the enemy. Grimoire boosts the chance of status effects working. Death Call kills all muted/stunned enemies. The result? Instant death to anything that isn't immune to Curse/Mystic.
Pierce bypasses resistance to, nullification of, and draining of physical attacks, unless the demon's name starts with "Bel" and ends with "dr". Pair this up with Deathbound and you can demolish bosses that would otherwise make a physical assault hopeless, such as Belzeboul and Naoya.
That’s only scratching the surface of the sheer game breaker of pierce and melee in general for true game breaker effect combine deathbound and pierce with ares aid(critical rate up 50%)and phys jump(physical damage up 1.5X) either one cracks up the damage a lot(especially ares aid) but using both allows you to kill most day 7 bosses in one turn if you have full health(you have mediarama for that) even if most demons can only inherent 2 passive skills your psychical based character alone should be enough to wipe out most of the boss health so that your demons can finish the boss off.
Beast and Kishin demons on the same team. Beasts let you move again after you attack, and Kishins let you attack twice.
Even better, try Kishins and Vile / Dragon type demons. As long as your opponent doesn't have a demon of the same type, you can attack for two turns without the other team even touching you.
Vile and Dragon do come with a drawback though, of making your character wait longer between turns and reducing your Move (Viles have a higher attack range, but make you even slower). Still, that's not a big deal when you can attack twice in a row without ever being hit back, pretty much decimating whatever you're attacking.
In Devil Survivor 2 higher level Dragons get Evil Flow. One usage of this buffs your attack range to 6, allowing the player to attack anything shy of a few bosses with complete impunity.
The Multi-Strike skill introduced in Overclocked is a sterling example. Hits the entire enemy team with up to seven hits each depending on the user's agility. Stick it on a physical specialist with decent agility like Atsuro in the original or Hinako in the sequel and you'll quickly end up with a one-man/woman wrecking ball capable of completely decimating 95% of the enemy teams in the game.
Why would you want to decimate, taking out one demon? Use it to straight up destroy. Although, if you really want to decimate, Multi-Hit would work better.
Better yet, mix it up with Drain Hit and Pierce. So long as there are no enemies that repel physical attacks, the HP recovery you get after every use will allow you to spam it infinitely.
Mixing Passive Skills can lead to this. A lethal one is a variation on Keita's strategy: combine Rage Soul with Twin Strike, Attack All and Ultimate Hit. See how many enemy teams survive that. Other variations include Grimoire, any multi-hitting physical attack, and any ailment skill.
Titania, Purple Mirror, and Metatron in Devil Survivor 2 are immune to all but two elements. Fusing them with the exact right passive abilities makes them invulnerable to everything but almighty attacks. Use the Release Passive add-on to add Hero Soul (take attacks for the leader if your resistance is higher) and their leader is safe from a good 80% of attacks.
Demikids Dark version Fusion system that affects only stats. While its not as unique as the Light version classic system, this is arguably better. With some easy work, and extremely abusable battle arena that gives out mid and high level demon as a prize, this allows you to Elite Tweak your party to ridiculous level.
The skill giving item. Oh gosh the skill giving item. Simmilar to Updated Re-release of Persona3 this allows you to give skills to your demon, except your option in this game is limited to some exclusive moves instead of almost every moves in the game. The catch? One of these items that is alvailable pretty early gives you 8 randomly hitting move that is alvailable to pretty much every demon in the game with fairly strong base power and sometime extra effect. This basicaly renders every other move barring healing move completely obsolete since you can spam this move to win any kind of fight. Combine with the fusion system above and even the Bonus Boss wont last more than one round.
If it has Victory Cry, expect to use it a lot; Victory Cry restores the user's HP and MP after every battle, eliminating the need to restore its MP with expensive or hard-to-find items. If it also has a high-end spell like Megidolaon, expect to mop up the final dungeon with it.