His V3 is (or was, at least) bugged; his forms are supposed to have separate HP totals, but they don't. One effect of which is that once you inflict enough damage to any elemental form to get him to discard it, he'll already be low enough to discard all the others as well and so won't attack you. This makes it very easy to kill him before he hits his final form (and that form still only has 200,000 HP, which by then your boss-killers can easily do far more than per hit, so even if he does reach it, he's still screwed).
Breather Boss: Pretty much every boss on the 19th floor, given what you had to go through on the 18th.
Game-Breaker: Most attacks with a high damage multiplier and/or that can ignore the enemies' defense, such as Nitori's Megawatt Linear Gun or Eiki's Last Judgement. Some attacks, however, may seem like this, but are balanced out by some weaknesses (Rumia, for instance, has both kinds of attacks, but her average magic stat does her no good).
World-Shaking Military Rule + Yukaris Spiriting Away can pretty much trivialize most boss fights. The former buffs all stats for frontliners except speed to 100% and the latter puts all frontliners timebar except the casters to 100%, allowing them to act again. If you can set up a rhythm of switching where you can use the latter move in a quick succession, you can deal a large amount of damage to an enemy in a short time.
Goddamn Bats: Any random enemy with absurdly high speed or high attack, due to how the game punishes you for finishing a fight with less than full health.
That One Attack: Djinn Storm, which depleted your entire party's SP, including the back row.
That One Level: Floors 10-12 are a nightmarish 3-floor lock puzzle with an insane amount of backtracking. Floor 7 is also notable for being a frustrating teleporter maze.
Floor 18 features a one-way maze. There is only one set path that leads you the boss. Any other of several branching paths will force you to the start of the level. Even if you manage to uncover all of the paths, you better move slowly. Otherwise, you might go the wrong way and there is no turning back unless you walk all the way to the beginning. Oh, and enemies on floor are annoying and tough to kill. Have fun!
Labyrinth of Touhou 2
Breather Level: The 13-15th floors are filled with enemies that are mostly weak to Cold and are not particularly threatening. The encounter rate is significantly lower compared to previous floors as well. Make sense since these floors focus on temperature puzzles instead.
Game-Breaker: Nitori again in the second game. One of her special skill is called Maintenance which doubles the bonus of equipments. This includes bonus from ailment resistance, and elemental affinity, solidifying her defenses to tank-like levels and making her hit like a truck, that is on top of having Super Scope 3D, the second high-multiplier attack in the game.
Parsees Jealousy of the Kind and Lovelys damage ties with Nitoris Super Scope 3D. While it requires enemies to be inflicted with Terror to be able to do damage, Parsee has the skill Final Blow, Jealousy Manipulation, Flame of Jealousy which greatly increases the attacks damage to absurd level. The fact that most of the enemies in this game are weak to Dark also helps. Given the right setup of the above skill, you can even kill the final boss during its first phase.
Hina has Curse Reversal, a skill which treats all debuffs inflicted to her as if they were buffs. With her Biorhythm spell, which debuffs all allies and enemies, she can bring her all of her stats up to +50% quickly, making her nigh-immune to most enemies attacks. She also has Pain Flow, which discards all of her debuffs and deals a heavy damage to all enemies. In addition, it also deals an additional damage depending on the number of debuff discarded. With Biorhythm, Pain Flow will also deal an absurd amount of damage, especially if you invest all of her bonus stat in Magic. She also have a great synergy with Parsee, another Game Breaker, who has Jealousy Manipulation which powers herself up per ally or enemy currently debuffed.
The subclass Diva allows the character to constantly push back the turns for every enemies and all allies except the caster back. Repeated usage will cause everyone else to have their turns shifted to 0. While this may sound useless, if you have someone that is fast enough to outpace the enemies, they will get their turn before the enemies and before the Diva pushes back everyone's turn again. Done right, you can deny the enemies to make a single move for the entire battle. This is usually done with or involves Aya who sports the highest speed in the game as well as having a skill allowing her to immediately give someone else a turn.
That One Attack: Detonation, used by a handful of enemies, deals damage to frontliners equal to its HP minus 1. Unless you reduced its HP (thus lowering the damage) via attacking or got lucky with evasion, prepare to be sent back to Gensokyo.
World Devouring Destruction is just like World Devouring Calamity/Djinn Storm, except it also reduces your frontliners' HP to 1.
Black Universe reduces your character's HP to 1 on top of inflicting Paralysis, Terror and -50% Speed Debuff, which guarantees a kill if that character is not healed in time.
Half Moon Slash halves the HP of all characters on the frontline, unless they evade it. In addition, since the game rounds down HP instead of rounds up, if a character is hit when his/her HP is 0, they are instantly defeated. Combined with Black Universe, losing a party member is pretty much guaranteed.
Dividing Slash is considered the most annoying attack of all. It is a Dark attack that can inflict Poison, Paralysis, Terror and Silence, severely crippling anyone unprotected. Not only has it the highest damage formula in terms of enemys attack but it also has only 7000 post-use delay, meaning enemies can spam this attack quickly.
That One Level: Floors 18 is filled with fast-moving enemies that either hit hard or can quickly cripple your frontliners. Unless you have Aya or characters with good speed, your party will be torn apart in no time.
That One Sidequest: Recruiting Suika. It requires you to buy and give her 3 different kinds of Sake in Nitori's shop. Each Sake progressively costs you a lot of money. The first one requires 3k, second, 12k and the third, 36k. That's a total of 51k money wasted to get her. Money is best spent on boosting characters' stats or buying useful items. It basically forces you to grind heavily if you want to recruit her and maintain your party's strength.
There are a number of events that require several certain characters to have a certain amount of Battle Points in order to recruit a character or fight a boss. Some can take around 200-400 BP so you have to spend a good amount of time defeating enemies on the first floor if you want to quickly gain BP. The worst one is recruiting Flandre, which requires all the other SDM crew to have 300 BP or move so you have to have them stay in the frontlines for 100 battles if you want do this the quickest way, which gets tedious really, really fast.