Varin Omega somehow manages to be this and the Climax Boss. While his first form is somewhat difficult, those who fought Ongyo-ki in Shin Megami Tensei III: Nocturne will be familiar with how it works, and the only thing that makes his second form a challenge is Hunger Wave. Other than that, it's a rather simple fight.
The Final Boss of the second game, Brahman, is very impressive looking and has five forms. All of these forms go down very easily, and while they do get harder, it's easy to counter most of his attacks. This is likely due to the sheer pain provided by the preceding boss, Meganada.
Complete Monster: This duology's Serph Sheffield, when placed in charge of the Karma Society's God Project, began using everyone and anything around him in his quest to attain the power of God for himself. He uses Sera's growing love for him to push her Psychic Powers into further maturation, only to express disgust at Sera's creation of a virtual paradise consisting of AIs she uses to escape the pain of his experiments. When Colonel Beck proposed to change Seras paradise into a training simulation for creating stronger AIs, Serph overrides Heat O'Brien's warning against destroying Seras mental condition and tells Beck to proceed. After a desperate Heat pulls a gun and demands the project to stop, Serph tells Heat that a human's mind is no different than a machine, and that his subjects, including Sera, were expendable and could be replaced, before manipulating Argilla into killing Heat. After being turned into a demon by a heartbroken Sera, sending a massive data surge into the now black Sun, Serph goes on a killing spree starting with Beck and Argilla before being killed offscreen. Refusing to accept that he is dead, he re-appears at the EGG facility as Solar Data and tries to kill the AI Serph and Heat while proclaiming to have the power of God. A cruel and manipulative young man, his actions are largely responsible for the disasters plaguing the series.
Ensemble Darkhorse: Heat was this to the Atlus staff at least during the first game. He's fairly popular among fans as well, thanks to his Axe-Crazy disposition.
Epileptic Tree: Satan in the second game states that a "heretic is trying to destroy the world." Fans theorize that he's referring to either Aleph from Shin Megami Tensei II or the Demi-Fiend from Nocturne.
The Seraphim appearing in the second game really don't help any. Expecially since one of them: in a factory: says "This is not the factory I knew..." Guess what one of the areas was in Shin Megami Tensei II.
Before fighting Seth in the second game, he says that "you are not the one who is to judge." He's most likely referring to Zayin, who Seth fused with to become Satan.
Brahman has something akin to the Amala Drums on the heads of his various forms. Let that sink in for a minute. Though it is confirmed inseries he is not connected to God/Great Will by Schrodinger himself.
Some theorize that the Demi-fiend's dying message is a response to the game over screen of Nocturne.
Putting all of Serph's stat points into a single stat can lead to some hilarious results, especially Magic. He'll be a Glass Cannon, but his spell damage output will quickly become much higher than enemies and bosses of his level are designed to handle, enabling him to blast through them in a few turns. The Magic stat also raises MP, giving Serph way more than he ever needs and allowing him to top up the party's health with basic Dia after every battle at next to no cost, meaning you won't need to rely on Heal Terminals or the Level Up Fill Up for most of the game.
Cocytus in the first game. True, it's obtained from a Bonus Boss, but it's one that can be defeated fairly early on. It hits all enemies multiple times for decently-high Ice damage and has a very high chance of Freeze. And frozen enemies always get criticaled by physical attacks. The freezing can even affect some enemies that are immune to ailments, too. Combine this with a Magic-optimised Serph and Mind Charge...
Zig-Zagged with Debilitate. It's practically required for any of the Bonus Bosses (and they'll use it on you as well), but if you know the Mantra grid you can go for it pretty early on, and it makes a lot of storyline bosses a complete joke. Especially since bosses don't start using Dekunda until the endgame.
Goddamned Boss: Varin Omega isn't that hard, but both his forms have annoying luck-based gimmicks. For his first he's invisible, so you need to target one of 6 empty spots and hope you hit him. Sera gives you hints on where he is, but she can only narrow it down to two spots at most, meaning you still have to go through at least a coin flip before you can damage him. Thought you could cheese it by using spread moves? Nope, they automatically miss. This effectively renders this form immune to debuffs too, as all of them are spread moves in this game. His second form has Hunger Wave, detailed under That One Attack below.
It Was His Sled: The Bonus Boss fight with the Demi-Fiend in the first game is so infamous that few fans call it a spoiler anymore.
Late-Arrival Spoiler: The mysterious "Angel" that kicks off the plot of the first game is actually a woman named Jenna Angel. She only appears as a disembodied voice until the last boss fight, but she's shown as herself from the beginning of the second game, and the game assumes you know who she is.
Les Yay: Pretty blatant in regards of Argilla and Jinana. Heat even Lampshades it by complaining about how she's upset about her "girlfriend". This was due to her being forced to kill a frenzied Jinana.
Like You Would Really Do It: Averted. When you first see Roland and Argilla'sHeroic Sacrifices in the second game you'll almost certainly dismiss them as just hiding because the game still has a long way to go and, given the way you're encouraged to optimise your characters for specific roles, killing 2 of them off for real would SERIOUSLY handicap your party's effectiveness over the long term. By the time Gale and then Cielo have been far more conclusively killed off on screen it finally sinks in that, no, Anyone Can Die- as you immediately see when Serph and Sera, the last 2 characters left, die after them and the final stage of the game effectively takes place in the afterlife.
Magnificent Bastard: Jenna Angel is pretty much at the center of every important event in both games. Taken to its logical conclusion in that she is both Sera's mother and father.
Memetic Badass: The Demi-Fiend has acquired this sort of status due to him serving as the first game's ever-notorious Bonus Boss. His successor Satan also has this status, but to a lesser extent that the Demi-Fiend.
The sound of the karma meter going up after battles, especially after getting a ton of Atma Points.
The Demi-fiend's death scream in the first game (which, given Nocturne, is the only time it's a good sound) and all the sounds Satan makes while dying in the second game, including his scream and the solar data leaking out before he explodes.
Moral Event Horizon: Even after all the went through, the heroes are still disgusted by Earth eating his teammates for strength in the second game.
Narm: Cielo's Jamaican accent. It was intended to emphasize the friendly, outgoing disposition of the character but most people just found it a little too ridiculous.
Scrappy Mechanic: Berserk Mode in 2, which happens during higher numbered Solar Noise phases. You can't use magic or items and everyone becomes a Glass Cannon. The only thing you can do is use physical attacks (or Hunt skills) or wait a turn, resulting in Critical Or Death battles. Fleeing is also an option but Karma points get more than doubled if you win.
Thank god fleeing isn't just an option, but has a 100% guaranteed success rate in Berserk Mode. Also, the developers applied Fridge Logic to the idea of only being allowed to use physical attacks, so your characters ignore enemies' physical resistances and immunities, allowing them to slaughter enemies they wouldn't even be able to TOUCH with physical attacks normally.
Signature Scene: Despite being a sidequest that has nothing to do with the main story and requires slogging through a large part of a New Game+ to get to, the Demi-Fiend's notoriously difficult boss battle in the first game is one of the most well-known parts of the series.
Tier-Induced Scrappy: Cielo in the first game, due to his iffy stat distribution and weakness to all status ailments. This is possible to fix with Null Ailment, but this is usually only obtained very late in the game. He is Rescued from the Scrappy Heap somewhat in the sequel due to better stat distribution.
As mentioned above, Varin Omega's Hunger Wave is the only reason his second form isn't a total joke in the first game. It's also not a particularly threatening attack, just very, very annoying. It inflicts Hunger status on a random number of party members. Hungry characters will spend their turn either doing nothing or attacking an ally. (And if you just Power Charged or used Tarukaja...) The only cure for Hunger status is Sera's song, and she can only sing for one ally per turn. And even then, who she sings for is random. So you can be critically low on health and Sera decides not to cure your healer. And if all three allies get hit with Hunger, you're not doing anything productive for a while. Harihara also uses it in the second phase of the battle against her.
Got Satan in the second game to extremely low HP? He uses Diarahan. Gate of Hell in both games counts as well since it hits everyone and has a chance to turn you to stone.
That One Boss: Cerberus in the first game, Meganada in the sequel. If you beat Meganada, even just barely, the final boss will be cake.
Let's not forget Ananta. It gets 7 actions per turn, hits you with Dekaja, boosts itself through the roof with Tarukaja and Power Charge, and is smart enough to attack the same person constantly rather than spreading out the damage. Not to mention that it likes starting every turn by casting Sonic Wave, Stun Wave, and Dream Haze in succession. And if you even try to use a Void Physical ability, it rips of Mudo and Mudoon for every attack. No elemental weakness either.
Anyone remember Vasuki? This thing uses both Maziodyne, an electric spell that can shock your characters, and a unique Ice spell that has a good chance to freeze your characters and, barring crazy preparation, you can only guard against one element and Vasuki is smart enough to attack with whatever isn't blocked. He can also toss in two unblockable Megidolas using the two actions that he gets per turn. Oh yeah, and he's encountered earlier than Ananta. Your only saving grace is that Vasuki is weak to fire, but he has a lot of health and has ailment attacks ready if you brought Cielo along.
That One Level: The first floor of Co-ordinate 136, less for difficulty and more for it being just plain frustrating. You have to navigate a maze of portraits to find the portrait of the "good prince", but walking in front of a portrait of the "evil prince" (read: every portrait that isn't the good prince) drops you in a pitfall to the dungeon, forcing you to slowly make your way back to the beginning. The problem is that each room has three exits, which are one-way, and often two out of three exits will drop you right in front of an "evil prince" portrait with no way to avoid it. Making it even more tedious: each individual "evil prince" drops you into a different room of the dungeon, rooms that can only be accessed by intentionally falling... and some of these rooms have treasure chests in them. Yes, you have to fall for every trap, slogging your way back each time, if you're a completionist. Oh, and the Encounter Repellant isn't available at this point. The good news is the puzzles on every other floor aren't anywhere near as bad, and are actually quite well-designed and fun.
Woolseyism: Mostly kept to name changes: the Assignments became the Vanguards, the Merrybell became the Maribel, "Mick The Nick" became "Mick The Slug", and so forth. Strangely enough, they didn't change the names during cutscenes: Gale's map of the Junkyard displays all the original names of the Tribes and Tribe leaders.
They named one of the Mantra (skill-teaching equipment) "Wikipedia" as a joke.