Although Goopy Le Grande is a Breather Boss, his second phase is when things get difficult. After getting bigger, Goopy bounces much faster than he did in the first phase. He also has the ability to actually punch you with his boxing glove. There are very few safe spaces and extra good reflexes will be needed for this phase.
Probably the earliest horrible attack you'll face is Ribby and Croaks' slots when they roll Tigers, which takes the form of several spinning platforms with spikes at their sides and bouncing balls at their tops coming at you at high speed. There are very, very few safe spots during the whole thing and really good reflexes will be necessary to make it through.
Among Baroness Von Bon Bon's many minions, probably her deadliest is the innocuous little jellybean soldier that sprints out from the gate at random intervals from the second phase onward. It's so tiny and moves so quickly along the ground that you're almost guaranteed to overlook it and get hit by it the first time, and probably at least a few more times afterward because it's so hard to notice when all hell's already breaking loose around you.
Beppi the Clown:
The penguin clowns that shoot baseballs at you in the final phase. More specifically, the actual baseballs, due to their sniper-like accuracy and tendency to trap you in bad spots.
Many people are also irked by that goddamned rollercoaster train in Beppi's second and third phases. It's fast, enough to pretty much always catch you if you're on the right side of the screen (and it covers what would otherwise be a safe spot from Beppi in his third phase), and while it does have a tell, it's easy to miss since it's small and part of the background. It gets noticeably faster for the fourth phase, but it's not as bad then since now there are platforms that are safe from the train.
Fireball bubbles, tossed out by Grim Matchstick's final phase. Their patterns are troublesome enough, but they also force you to hold your fire, or else you'll accidentally pop them and release several more projectiles. As soon as they start getting spammed, either your damage flow suffers or you'll get caught in a fiery nightmare.
Djimmi the Great can start his battle with one of three attacks he'll use until the phase is over, and one of these is an order of magnitude worse than the other two: The cats. Cat sarcophagi, to be specific, that split open and send out several homing cat scarab projectiles, while the sarcophagus halves remain harmful, resulting in a screen full of pain in a matter of seconds. People are known to keep restarting the fight until he pulls a different trick, as the flying swords and even the jewelry toss are much less likely to result in a terrible start.
The barrel in Captain Brineybeard's battle. Like the jellybean soldiers, it's easy to forget about this attack because it hovers just outside your peripheral vision and you have to worry about the other attacks coming towards you. And unlike the jellybean soldiers, it stays the entire battle instead of one or two phases. In addition, it's indestructible, but it still attracts the Chaser's bullets, making that weapon practically useless in the fight.
Rumor Honeybottoms' triangle attack during the second stage of the fight. The triangle itself deals damage and Rumor likes to summon it right on top of you or above you, which becomes problematic when you need to move up due to the nature of the fight. Even if you can move out of the way, it still moves in your direction and fires three rounds of tiny triangles from its three tips, of which the first is fired almost immediately. Every attack sequence that involves these always has you deal with two of them summoned after each other as well. This attack is the #1 reason to bring the Parry Sugar to her fight, as it's one of the few truly reliable ways to shut it down (everything involved in the attack is pink).
Cala Maria:
Her Ghost Pirate summon. Their homing attack is very quick, and they are rarely the only thing on the screen. If she uses it while her pufferfishes are active, dodging it is particularly difficult.
Her stare that turns you to stone. Better hope a projectile or cave obstacle isn't heading towards you when you get hit by it.note Funnily enough, in the second phase, the best counter is to get as close to her as possible. The beam fires a good distance from her face, leaving just enough of a blind-spot for you to swoop in and pelt her with bullets and bombs... until the electric eels show up, anyways.
Sally Stageplay's Big Wave attack in her third phase, due to being impossible to avoid if you miss the parry star, bounce off of it too early, or don't equip the Smoke Bomb charm (which lets you dash through enemies without taking damage).
Dr. Kahl's gems of death during his final phase, which bring true Bullet Hell into the game in a segment where electric barriers, which hurt you and stop your shots, but not his, are already closing you in, not to mention they never stop coming until you win or die, are probably responsible for more deaths than any other attack in the game. "Got the perfect equation to hinder yer evasion" indeed. Luckily, they've been nerfed, with actual parriable pink projectiles being shot out along with the usual death-inducers, so the load is at least lightened.
While Hopus Pocus' rabbit skull attack can be cheesed with the Smoke Bomb, if you didn't bring it into King Dice and land on his space then you're in for pain, as you have about half a second to spot the gap between the skulls and jump/dash/drop through. If that gap happens to be diagonally beneath you, the attack is basically undodgeable.
King Dice's first initial attack may catch some players off guard, since he's the only boss who doesn't wait until the fight introduction is finished before attacking and his hand is likely to slap you down. The rest of them aren't quite as bad, but there's some minor Hitbox Dissonance with his marching cards due to their boxy shape and deceiving size, and thus parrying them properly can be quite erratic until you get used to it. Even without the issue with the hitbox, the card attack is impossible to dodge unless you have learned to time your parries perfectly.
Most of the Devil's attacks are fairly easy to predict and dodge after a go-through or two... except for the fire spell. When his eyes light up and that pitchfork starts spinning, only the Random Number God knows which of the three fireball patterns he'll be tormenting you with, and all of them are insanely fast and screen-crowding.
The Delicious Last Course:
The Moonshine Mob have a very devious surprise near the end of their fight. They fake out after the third phase, tricking the player into believing they had won the fight, only to reveal that the Snail Boss is also a fighter and begins to shoot the player. Not too bad, but considering many players have already experienced the trap first-hand and usually die because they only have one health point the first time around, it's a very mean trick to say the least. Granted, this attack only works once to the unsuspecting player, and it's relatively easy to deal with once the player is prepared.
The Howling Aces are generally seen as one of the easier DLC bosses, but the entire third phase can be considered this to some thanks to the screen rotating 90 degrees anticlockwiseevery time the airship opens or closes its mouth, making the attack with the dog bowls (when the mouth is open) harder to dodge than it looks thanks to you being sideways, and the lasers (when the mouth is closed) problematic when you're upside down. Fortunately, it's possible to get an easier third phase that lacks this interface screw if you spare the second phase's pups by getting all their health low without outright defeating any of them.
The geese in Glumstone's first phase. The very second he summons them, the entire top half of the screen becomes flooded with hostile waterfowl. While you can avoid them by escaping to a platform that's currently been lowered, you're completely at the mercy of the RNG and have to hope that it doesn't suddenly raise you into the flock of geese. The sheer unpredictability of the platforms and how they interact with the attack make it one of the most irritating moves in his arsenal, by far.
Esther Winchester's second phase makes Dr. Kahl's Bullet Hell look easy as pie by comparison. As soon as the first phase of her fight is over, the second has her grabbing a vaccum and sucking up everything (including money) and then throwing it at you with the vaccum. This would be too much of a problem except much like Dr. Kahl, the sucking never stops until the third phase when Esther gets turned into sausages.
Chef Saltbaker:
Out of all his attacks in his first phase, easily his most infamous is the Desert Lime boomerangs. Not only do they swarm the screen just as much as everything else he throws out, but each one of them will change directions and height once they hit the end of the screen to try and go for another hit. Already annoying to deal with on their own, but combined with everything else he throws onscreen, they can easily lead to a good amount of deaths. And he has a chance of starting the fight with this attack almost instantly. The good news is that they have a somewhat predictable pattern, always coming in low and going out high, or vice-versa.
The yellow fireball bouncing around during his first two phases is also despised for how badly it complicates his fight. It's a Painfully Slow Projectile that homes in on you, and travels in a sweeping arc. While it shouldn't be too hard to dodge in theory, it makes it way harder to avoid the limes, animal crackers, gnome berries and pineapple leaves since you're often put into a position where you can dodge one but not the other. Plus, it's simply easy to lose track of it in the chaos of Saltbaker's insanely hectic fight. Expert mode ups the ante by adding another one into the first two phases.
