Awesome Music: See the Awesome Music page for the series. If there's one thing everyone can agree on with this game, it has what is considered to be the best Soundtrack in the entire series so far, with the only real vocal complaints being over whether if Bowser's Inside Story had a better final boss theme or not. (And even then, people agree that regardless, the final boss theme for this game is still utterly amazingm as well.)
Badass Decay: Bowser X, the Bonus Boss for the last game had a lot of HP along with many unavoidable attacks. This game, since Bowser is an early-game boss, Bowser X is easily curbstomped with a Luiginary Attack, assuming you fight him later on in the game.
Justified in that you're fighting powered-up memories of the real Bowser as he was at that time. The real Bowser ends up averting this by hijacking the endgame.
Antasma. Some fans of this game prefer him over Bowser by a longshot and would've much preferred him to be the final boss instead of Dreamy Bowser. The fact that he has arguably the best battle theme in the whole game helps. Others criticize him for his rather lacking characterization for a Mario RPG Big Bad. And people who find him Rescued from the Scrappy Heap upon finding out that he's not the Big Bad meet opposition from people who preferred him over Bowser (and thought he was gonna be the Big Bad).
Bowser also has this status. Some fans are happy to see him as a powerful and somewhat intelligent, Badass, menacing villain once again. Others criticize him for betraying potential new fan-favorite villain Antasma and hijacking the plot from him, and believe that such portrayals of Bowser should be confined to the main series games rather than the RPGs. There's also the fact that Bowser has lost his humorous Ineffectual Sympathetic Villain portrayal from his other RPG appearances in exchange for straightforward villainy.
There has been some complaints over how Dreamy Luigi was handled. In a game that was suppose to be celebrating Luigi, many have noted that Dreamy Luigi ends up feeling more like an item than a partner in the dream world, thanks to serving as a powerup for Mario and being used mostly for special attacks. Many have stated that, ironically, Mario is the one who ends up feeling like the main focal point of most Dream World battles, thanks to being the only one who fights conventionally and who you end up controlling for the most part. On the other hand, many feel like Luigi still gets a good amount of focus, and his special attacks in the Dream Worlds help him stand out and really give him a chance to shine after being the Butt Monkey for so many years.
Best Boss Ever: For regular battles, Wiggler and Popple, Bowser Jr, and Dreamy Bowser. For Dream World battles, the Elite Trio, Kamek and Antasma. For the giant battles... Well, if you don't hate them, it's agreed that they grew the beard with Earthwake, with him and the later ones contested for being the best.
Best Level Ever: Mount Pajamaja, a climb up a huge Hailfire Peaks-themed mountain topped off with a challenging but fun boss battle and gorgeous music for the snowy portion. Additionally, many of the game's Funny Moments are here thanks to the Massif Bros., the Muscle Lord statues, and the Mega Pi'illos.
Big Lipped Alligator Moment: During the battle with the Smoldergeist, Starlow randomly casts a gust of wind to blow out his fire. The Mario Bros. are stunned for a few moments, then the battle resumes as normal and neither that moment nor Starlow's ability is ever brought up again.
Antasma is somewhat easier compared to Giant Bowser before and Dreamy Bowser after. This makes sense from a narrative standpoint, considering you fight him right after Bowser hijacks the plot from him.
If you can count them as a boss (based on the music), then the Fly Guy R thieves are this. They're just normal Fly Guy Rs. No new attacks or gimmicks, and no more health or higher stats than usual. And you've beaten up dozens of them already.
Possibly Bowser Jr. He's not super easy, but his attacks are also not that difficult to dodge, especially compared to those Bowser X in the last game. He's a fun boss to fight though, so one could say he's the one example of a Bonus Boss ever who's actually legitimately entertaining to battle and isn't just ridiculously hard for the sake of being ridiculously hard. And then there's the Catharsis Factor.
Popple is fought right after beating the Wiggler; in the exact same fight, nonetheless. He is much weaker than the Wiggler and has one easy to dodge attack, so he shouldn't put up much of a fight.
Breather Level: The entire Ultibed quest, at least in terms of the part finding. Mini games, basic enemies, already visited areas, not much that's actually difficult in the slightest. Which is nice, considering it comes right after Driftwood Shore and its dream world, and right before Wiggler, Popple, Dreamy Wakeport 2, Earthwake and Somnom Woods, all of which are much more difficult to beat.
Admit it: Chasing down Bowser Jr. by hijacking his Clown Car when he's out of it is immensely satisfying! The fact that it's an inversion of how things usually go when a chase sequence is initiated is the icing on the cake.
Fighting Popple, who Took a Level in Jerkass for this game, is also immensely satisfying despite (and perhaps because of) being incredibly easy. It's really too bad that he's not in the Battle Ring.
Cocoknights (and their R equivalents), Mechakoopas and Beehosses. They've all got at least one attack that's infuriating to counter, fairly decent stats and come in large groups in the endgame. Have fun.
Most of the Dream World's enemies are Demonic Spiders. Special mention to Lob-omb (and their R variant) with their attack where they throw one big bomb. Sometimes, it's thrown next to you or behind you, and predicting that and dodging the attack is really diffcult because it goes so fast. And it deals a huge lot of damage.
Pi'llodactyls start out in the background, the first enemies that you need a Taunt Ball to engage. Their stats are ridiculously high compared to other enemies on Mt. Pajamaja, with three times their HP, high defense, and a fireball attack powerful enough to 2HKO and that still requires you to repeatedly jump off of the resulting burn spot even if you dodge it. At least it gets rid of the other enemies.
Dark Blocks have only two attacks, and one of them is not all that hard to counter. The other, however, requires precise timing to hit the block multiple times in a row, and if you don't hit it enough times before it finishes the attack, you will take a decent chunk of damage and possibly get dizzy in the bargain. It's a Dream World enemy, too, so you don't have a second character to bail you out if things get dicey.
So far, the Massif Bros (with their random beef references and muscle obsession) seem to be the most popular new characters in the game, and one of the things even some of the more harsh critic reviews call hilarious.
Kylie Koopa's (originally from Partners in Time) popularity alone earned her a spot in this game. Her optional quest in this game is also regarded as pretty fun in general.
Deviantart has been brimming with Brickle/Britta art as of late.
There are some who ship Antasma/Bowser, since they do get along. At first.
Among Japanese fans there's Bedsmith/Broque Monsieur, a Crack Pairing of extremes that manages to be ridiculously popular.
Franchise Original Sin: Antasma is criticised by some for being a Flat Character, but he really has no less characterization than most Mario RPG villains (see Smithy, Cackletta, the Shroob Princesses, Grodus, the Shadow Queen, and the Dark Star). The difference is that those villains either had underlings with more personality to pick up the slack for their flat characterization, or were aliens/eldritch abominations that didn't require much characterization. Antasma doesn't have either of those things to fall back on, so he stands out more as a flat character. It also doesn't help that he's being compared to the more popular RPG villains like Fawful and Dimentio, who did have more characterization to them. Many fans wanted him to be more like those two.
Birthday equips. These multiply a stat by 400% on your birthday (as set by the 3DS clock). Keep changing the date and you've got a ridiculously overpowered set of gear that can smash through anything.
Time freezing badges. Combine the Gold Badge and Miracle Badge, and you can stop time. And store the effect to use in any battle in the game. And use ridiculously overpowered attacks like the Zee Egg and Star Rocket twice before the enemy can even move. And then use the same effect again the second it runs out, for a max of four times in a row - or a whopping five if you time the Badge Meter's charging just right. Oh, and the Birthday equips stack on top of this. End result? You can literally do up to 50,000 damage before the enemy can even move or attack. It means you can take out the final boss before it can move, the penultimate boss before it can move and all four of Kamek's clones/forms before they can move. Oh, and it's possible to get this combo right before the final dungeon and completely steamroll through every boss in sight. And in the Battle Ring, the turn count does not move forward when this badge is in effect.
The POW Up+ rank bonus, which gives one extra POW point per level. While this may not seem like much, since damage and power do not have a linear relationship (try fighting some of the first enemies fought while at a high level), soon bosses will be defeated within a few turns.
The Challenge Equipment which becomes stronger the more challenges you complete, and eventually becomes some of the strongest, if not the strongest equipment in the entire game if you do all of the challenges in the game. This is further broken by the fact that finishing the list completely gets you the Duplex Crown mentioned below.
The Golden Equipment which has stats that are based on your overall coin amount, meaning the more money you save, the stronger it is. This eventually can become so strong that at max power it's only second to the Challenge Equipment seen above. This becomes extremely easy when you realize that you don't have to shop for items and weapons as much as you think considering that a lot of good ones will probably be dropped from enemies along the way, allowing you to save up money easily, especially with the Coin Gloves DX equipped.
Bottomless Gloves. Infinite items (aka your healing items never run out). In the Battle Ring, this completely annihilates much of the difficulty.
Coin Gloves DX and Gift Gloves DX. Using one of these on each of your brothers respectively means that both your chances of getting items (including rare ones) and your coin amount from enemies at the end of battle are significantly increased. The Coin Gloves DX are found by beating Madame Brioche's Luiginary Ball event which isn't too tough (at least compared to the other events in her arena) and the Gift Gloves DX are the rare drop from Bandits which are easy to beat, and come in groups, which make finding them pretty easy.
The Wellington/Farmer's Boots. What does this pair of boots give you? Why, the ability to occasionally get Beans instead of coins. Combine this with Dream World enemies and Luiginary Ball (and maybe the gloves that give you more coins from doing that), and you won't need to care about stats in the nearest future, as well as customizing the bros exactly how you want them.
Badge effect duplication badges. Don't be fooled by its description, the Expert and Miracle Badge combo has a completely different effect than what it says. It randomly copies a currently stored badge effect and stores it for later use. If there's only one badge effect stored, then that effect is automatically copied. It also charges faster than most of the other Gold or Expert Badge combinations. Now combine that with the time freezing badges, and you'll be able to quickly stockpile this effect, making the grind to get it much, much easier.
Try the Guard/Master Badge (Protects a Brother from 6 attacks) combined with the Risk/Silver Badge (Powers up everyone on screen). That should save you some time to attack the enemy with as much force as you can without worrying about getting hurt, like for example ... Luiginary Hammer, Luiginary Wall, Zee Egg, or Star Rocket.
The Casual Bros. Bonus power which halves your Bro Point (this game's version of Magic Points) consumption. There's an accessory called the Bros. Ring which does the same thing, and can stack on top of this effect as well which makes it so that one brother can literally use his Bros/Luiginary attacks for a quarter of the normal amount.
The Angel Bangle mixed with the Casual Bros Rank Up effect. The Angel Bangle restores 20 HP and 10 BP a turn. Casual Bros cuts the price of special attacks in half. None of the special attacks exceed 20 BP, so none of the moves with Casual Bros will exceed 10 BP. And since you heal 10 BP a turn, this means infinite Bros moves, with absolutely no restrictions outside of the Bonus Boss. In fact, the Angel Bangle in general is overpowered; even without Casual Bros., only 6 of the 16 special attacks use more than 10 BP, so one can still spam any of the other 10 every turn, or even one of the more expensive ones followed by a cheap one (say, Zee Egg followed by 3D Red Shell, Star Rocket followed by Dropchopper, or Luiginary Wall followed by Luiginary Ball) so that the combined BP cost still adds up to 20 or less. And then you get 20 HP worth of healing in the bargain.
You can use Bros. Boots as a substitute for the Angel Bangle. They recharge your BP whenever you use a Jump attack, based on how much damage you do. Sounds okay at first, but what's really broken about it is that the effect stacks with Bros/Luiginary attacks that use the boots, and given that those attacks do way more damage than jump attacks do, you'll often recover more BP with the boots than you used to perform the attack in the first place (especially if you have Casual Bros on like with the above example). Enjoy never having to use regular attacks again!
The Star Boots, which make any jump attack or special boot based attack recover health after you damage an enemy. Combined with the Angel Bangle, you'll probably never have to worry about health again for the most part.
Badge Charge Meter booster badges. It's possible to get this stacked up so high that you can fill the badge meter several times in one attack.
Silver + Miracle Combo. What does this do? Resets status to the last turn. But it doesn't reset how much you've beaten up the enemy, so they stay brutally mauled, you end up in a good condition again. This cures status effects and heals damage to the point you were at before, so it can act like a free combo of a Max Candy and a Refreshing Herb. For both characters. It's also very 'cheap', in that the bar for it is filled in just 14 hits (compared to more than 20 for most other powerful badges). And against Bowser Jr? It BRINGS BACK the Bros Attacks he steals! So you can go Star Rocket, Zee Egg, reset, more Star Rocket and Zee Egg! It can completely destroy the difficulty of the Battle Ring...
Boost Wear/Energy Wear. Seems useless at first, but filling your badge meter every time you get hit is a pretty damn powerful effect. Especially against the Elite Trio or Antasma, who have attacks that can hit about 10 times per turn. See how useful those badges mentioned above are? Well, they're even more broken when you can fill the meter every time the enemies hit YOU as well as the other way around!
The Slingsniper, despite not being the last Bros. Attack you get, has a special property to it that can make it way stronger then any of the other ones in the game if it's used properly. If you're fighting a large enemy, a star indicator will appear on them, and having Luigi score a direct hit on the star will cause the already strong attack to deal a critical hit for even more damage.
The Master Gloves. If you can get an Excellent on a Bros. Attack or Luiginary Attack, your BP is refunded. With this, you can spam the most powerful moves in the game with no cost, if you're good enough. Yes, that means you can spam Luiginary Flame/Hammer, Zee Egg, and Star Rocket to your heart's content. Combine with the already broken Gold + Miracle badge combo, and just see how fast everything dies.
The Duplex Crown, your reward for completing all the Expert Challenges. Equipping it on a bro lets him have two consecutive turns in battle. With that and all the other game breakers above, you're suddenly overpowered twice over. In fact, you'd probably be a little bit too hard on the game by then. Even in hard mode. In fact, it becomes almost trivial to beat the Battle Medley in the minimum 9 turns (1 for each battle).
Hermite Crabs. They're common, basically the only enemy in Wakeport outside of the Dream World, can't be killed with a preemptive strike and have hard to dodge, annoying attacks. You can't go four steps in some directions in Wakeport without bumping into one, and what's worse, their hard to dodge attacks? Have to be dodged ten times in a row for the 'Dodge Ten Wakeport' challenge unless you use Boo Biscuits against a single Virus in a single part of town. Oh, and they then get Hermite Crab R variants, which are also ridiculously common.
Fly Guys (and their R counterparts) take time to defeat and are constantly bringing in more Goombas to lengthen the battle.
The Elite Trio (Sergeant Guy, Corporal Paraplonk, and Private Goomp) can be rather annoying. None of them are terribly difficult to dodge or terribly difficult to defeat individually, but you can't beat them unless you take them all out in a single attack. Not in a row, on the same turn; any of them that aren't KOed will revive any that are, and they will happily combine this with a Mushroom or two to heal. Thankfully BP can be restored with badges and you can exploit Sergeant Guys cannon to damage them all at once.
Mt. Pajamaja is a Damage-Sponge Boss unless you take a turn to flip him over with the Hammer Uppercut. This is especially irritating in the Giant Battle Ring, as he is likely the only boss that you will run the risk of losing by running out of turns.
Ho Yay: Bedsmith's constant desire to "take a nap on" and "have a Pi'illo fight with" Dreambert has some very romantic undertones. The hearts that appear while he's saying this and Dreambert's horrified reaction to it says it all.
Some people feel Antasma is this after Bowser betrays him.
An interesting deconstruction are the Walker Guy enemies. They're tiny Shy Guys walking a huge Chain Chomp which basically is an example of a Jerkass Chain Chomp and a Woobie Shy Guy. In battle, it's less Shy Guy "walking" the Chain Chomp than "hanging onto the chain for dear life as the Chomp throws him around".
For the Expert Challenges, Monolift R can be this. They're only found in the area of Pajamaja that leads to the Pajamaja Rock Frame, relatively tiny compared to the main area. They also seem to have an uncommon spawn rate compared to Viruses in that zone, so if you accidentally kill it without getting the No Hitter or kill it on purpose thinking you can just find more, you'll have to backtrack there for the sole purpose of getting that challenge cleared.
Shy Guy and Shy Guy Airtub Rs are only found in just about the very last area of the very last dungeon in the real world, and have a spawn rate as low as or even lower than the normal Airtubs. Plus, the attack they launch when you hit them with a Taunt Ball is extremely difficult to dodge perfectly as it is much faster than the same attack with the normal airtubs, meaning you'll likely have to find at least two of them to get those last 10 points.
Magnificent Bastard: The main villain. Bowser finally gets his act together and plays the heroes and even Antasma to his benefit.
Memetic Mutation: Dreamy Brickle in a carrot suit and everything he says and does in it.
Most Annoying Sound: Mario saying "Here we go!" can get extremely annoying in a Giant Battle very quickly, despite it being likely to be heard only a few times on normal occasions. It is a reminder, though, that Rhythm Mushroom/Drill Stomp can be used.
Padding: Mole Hunt in general. You have to play this mini game twice, and there's not any real story related reason for it in either instance. Seems they just wanted a way to pad out Driftwood Shore...
The Dream Stone Spirit, considering how he acts like a total Jerkass to you and refuses to tell you where Bowser and Antasma were going with the MacGuffin, going so far as to turn into a giant drill machine to get rid of you for trying to get the information out of him. Needless to say, it will be immensely satisfying to beat the snot out of him as Giant Luigi.
Popple, for being a complete jerkass that initiates an annoying boss fight with Wiggler and becomes The Load during the fight, and then turns on you after you defeat Wiggler. Not to mention that his status as a recurring boss and his unique boss music is gone, so all of his likeable traits aren't around.
The Mole Hunt Proprietor. See that entry for Padding? Now consider the fact that you are only doing that minigame because this idiot wants to make a quick buck off you by withholding access to the next part of the game until you pay him. (Granted, 10 coins is a pittance by the point in the game where it comes up, but still...) It's as if Moneybags reincarnated into the Mario universe.
Ring puzzles in Somnom Woods. Major Guide Dang It, made worse by the lack of guides and ridiculously complex solutions needed. Made slightly easier if you wait before starting one, study the layout, figure out the path beforehand and take note of the shadows of the rings on the ground to better gauge their position.
Some of the Dream World mechanics are this, considering that Luigi's equipment has no effect on Dreamy Luigi and thus no effect on Mario when they combine in battle. Considering the fact you only control Mario, and only have one turn instead of two like in the real world, this makes most dream battles, and especially dream boss battles much tougher than they have the right to be. Also, since you control both bros, Dreamy Luigi can become a liability with the platforming if he gets left behind (and as a result, falls to his doom). The fact that Luigi's stats and equipment have no effect on Giant Luigi battles becomes a huge problem in later phases of the game.
The fact that Mammoshka doesn't always drop his rare item the 1up-Gloves at the end of his battle which has a 50 percent chance of happening as opposed to the normal boss rate of dropping their rare item 100 percent of the time. If you're unlucky and he doesn't give you his item after his defeat, you'll either have to fight him again and hope for the best, or his item becomes Lost Forever.
The gyro controls can also take some getting used to. It doesn't help that they are responsible for a possible Game-Breaking Bug in the game. Not only can they hurt you against the Zeekeeper but they're needed to finish off Giant Bowser. Even though the final phase is pretty easy, screwing this up causes it to repeat, turning it into a potential battle of attrition, and getting hit by one of Giant Bowser's attacks in this phase can be a one hit KO if your HP is less than 2/3 full.
The giant battles in general are this for some players. And there is no way to make them any easier; they are not affected by levels, stats, gear, or items, nor can you do them in Easy Mode after dying. The Giant Battle Medley gets special mention for forcing you through all five, with the only opportunity to heal being when Mushrooms appear during Zeekeeper X's egg attack or fall whenever Giant Bowser X drops into lava during the "Repel the Shell" segment. Unlike the Battle Medley, where the aforementioned levels, stats, gear or items will gradually allow you to defeat bosses before they can even move and recover from whenever you are hit, each Giant Boss has the potential to kill you no matter how far you've gone through the game.
Speaking of the giant battles...the Drill Stomp, aka the "sure wish I were ambidextrous" attack. Drawing circles on the touchscreen may seem simple enough, but you have to draw a lot of them (there is very little leniency), you have to do it three times in a row, the direction of the circle is randomized for each of the three rounds each time, and depending on whether you are left-handed or right-handed, one direction will almost invariably be much harder to do than the other.
The lack of any function to quit and reset to the title screen. (You have to go back to the 3DS's home menu, confirm, reopen the game, go past the 3DS logo screen, skip the pre-title cutscene, hit "Start Game", then you can finally select your game file.) Not having it makes certain things, such as fighting Gold Beanies, more tedious than they need to be.
The "Bomb Derby" bros attack. It's easy to get an "OK", but getting an "Excellent" is near impossible, due to the precise timing needed to hit all the bombs, and the fact that you need to focus on shooting them rapidly and hoping for the best, otherwise your EXTREMELY limited time will run out. Want the Duplex Crown? Too bad, you have to get an excellent in the Bomb Derby attack, which is even tougher than dodging Earthwake's hammer attack, the Zeekeeper's dimensional rift, and finishing off Giant Bowser.
Played with during the giant wiggler boss fight. Popple (who is on your side at the time) will "help out" by annoying the wiggler, causing it to get pissed and making its attacks harder to avoid. Once you get rid of it, he immediately turns on you so he can take the treasure for himself, implying he may have been doing it on purpose.
Players think that there are too many tutorials in the game. The player is still being given tutorials in Somnom Woods, the penultimate level of the game. The game (mostly) stops holding your hand in Neo Bowser's Castle.
Surprise Difficulty: This game, is by far the hardest Mario & Luigi game yet. It has bosses with a ton of health and healing abilities, difficult to dodge attacks and some actually very hard giant bosses and sidequests. And it gets worse on Hard mode. Thought you were in for an easy ride after Bowser's Inside Story? This game can and will destroy you.
One of the key factors is that, while other games in the series were easier if the player masters timed hits, this game is basically unwinnable if you can't figure them out.
Earthwake's hammer attack for starters, since it's very easy to fail to block, does a ton of damage... and so far, seemingly not one person on Youtube has managed to completely avoid it. Oh, and he swings about 10-15 times in a row on hard mode... Hint: Swing when you hear a chime.
Pi'illodium's hammer attack isn't much better. He slowly rises offscreen with a counter dropping down. It continues even when you can't see him, so you better know when it hits "0", and even then, be prepared for the speed at which Pi'illodium slams down on his target. Mess up the timing, take massive damage (and possibly dizziness).
Antasma's bat attack in his final battle (it's not so bad, but it's the potential secondary effect that makes it this: Antasma pulls Mario into a Dream Within a Dream and begins to chase him down while spitting dark flames, and Mario has to find the correct exit to wake up, the others contain spikes that can combo you AND won't pull you out of the dream), Pi'illodium's hammer transformation attack (hmm, every boss that becomes a hammer tends to become That One Boss), Bowser's chase attack in his first battle and a few others. Pi'illodium also has another one in the form of his self destruct move, which operates on a real-time timer in a turn-based game.
Mt. Pajamaja's charging attack is somehow easier to stop with the X version, as your sliding on the Touch Screen barely registers in the original battle unless you are practically slamming on it.
The Zeekeeper's dimensional rift attack. Even when the gyro controls are actually working properly, it's still a pain to avoid the energy balls AND his ramming attack.
In the Battle Ring, the attacks of the X version of Antasma all become That One Attack, because every single one has the chance to make you sleep. It's almost a relief that the boss who follows after is one of the most fun bosses to fight.
The Beehoss' bee swarm counterattack. When attacked, it fires a hard-to-dodge swarm of bees. Jump on the bees instead of jumping over them? They counter your counter with an unavoidable counterattack. This mechanic applies to their standard attacks, which are no easier to dodge.
The Mechakoopas' fireball march. In theory it sounds simple: bait them by moving into their path, then move back once they're done shooting. In practice, it's all but undodgeable, as there's no way to move out of their way and back quickly enough without getting hit by another Mechakoopa's fireballs. No, you can't just jump over the fireballs, as they constantly fire if you're in their path. There's a good chance they'll use this attack before you can even move, up to five times in a row. A way to dodge is to wait at one end of the battlefield until you see the exclamation mark indicating that the Mechakoopa saw Mario, then move to the other end of the battlefield and hop over the 1 or 2 fireballs the last Mechakoopa shoots at Mario, before jumping over the Mechakoopa itself.
Torkscrew can dig towards one of the Mario. Bros., completely submerge, then pop out a second later. You have to jump to avoid the drill, but if you're even a split second too early, the Mario Bro. will get caught on the drill, forcing the other to chase Torkscrew past several rows of bombs as the victim drops coins every few seconds.
Mammoshka's giant rock. It hits both Bros for massive damage and you have to hit it 3-5 times to get rid of it. Both Bros have to hit it at the exact same time; if one is slightly off, then the rock will crash into the other Bro for massive damage. That alone would be challenging, but the worst part is that it goes off-screen the 4th or 5th time it is hit. Unlike Pi'illodium's hammer, which comes down on a timer, there is no tell for when it's coming back down aside from memorizing the timing.
When Giant Bowser goes behind a fortress, one attack involves an army of Spear Guys running up to Giant Luigi to stab at his legs. You must tap on them to get rid of them, but doing so without taking at least some damage is virtually impossible, and the stabbing doesn't stop until every last Spear Guy is gone.
Chasing attacks in general. Bosses will occasionally force the Mario Bros through a long running sequence that requires constant evasion. Even those that end after a single attack connects tend to inflict heavy damage.
The Dark Blocks have the attack mentioned under Demonic Spiders above. They will show a counter (usually 3 to 7) and then start moving back and forth above Mario's head, and every time he jumps under the block and hits it, the counter will decrease by 1. If you get it to 0, then you have successfully countered the attack. If you don't, then the block will slam down on Mario's head for a good chunk of damage as well as the chance to inflict dizziness. And hitting the block at the right time is a lot harder than it looks.
Dreamy Bowser has two of these. First is a hammer attack where he hits one of the brothers (signalled by a coin flip) at a very fast speed. If you counter he will then hit the other one and it does huge damage to both brothers. The second has him summon dreamy versions of enemies though these are not the main problem as while they come in numbers rarely seen outside the dreamworld they are easy to beat. The problem is Dreamy Bowser goes into the background and heals while they are on the field.
The Zeekeeper. Due to how giant boss fights work in the game and since this boss is constantly fleeing from you, said act of fleeing and its attempts to shake you off are technically his turns, meaning you have to wait a while before getting to do anything of your own accord. To make matters worse, he's the only giant boss in the game that attacks before you get a chance to. It only gets worse when it begins to travel dimensions, instructing you to move with the gyro controls. Due to a fairly common Game-Breaking Bug, the gyro controls can often fail to register, making this fight impossible to win unless by sheer luck you can dodge its attacks by not being targeted. A lot of players likely haven't noticed this, but in order to actually catch up with the Zeekeeper, you have to dodge/counter his attacks perfectly just to get a chance to successfully attack him, and if you get hit even once you actually fall back a few steps and the Zeekeeper will just move on to his next move, one after another. What this means is that this battle can potentially take forever to get done with if you aren't perfect at dodging and/or countering.
Earthwake. Giant hammer formation attack causes hell if not countered properly. And it comes right out of nowhere after a simple fetch quest too...
Giant Bowser, because he's by far the most complex giant boss in the game and introduces a ton of new mechanics you need to learn/attacks you need to dodge or counter. Its gyro phase is arguably even harder due to a common Game-Breaking Bug mentioned above. Bonus points for the second gyro phase involving precise timing to get around a sequence of fireballs, because if you don't do this perfectly, you will fail and Bowser will get to keep attacking. And this is the only way to attack him at this point in the battle.
The second and final fight with Antasma. Not only are all of his attacks hard to dodge, but they also have a chance of putting Mario to sleep and send you to his nightmare realm, forcing you to avoid Antasma's fireballs and going into one of five holes to find the exit. Later into the fight, he will pull out Luigi and put him into one of his Antasmunchies and will rotate them to change their formation, and you might JUST accidently glance at the wrong one and probably waste a turn just to free Luigi.
Dreamy Bowser, which continues the trend for HARD final bosses of the Mario & Luigi series. For starters, you can do no damage to his actual body until you can defeat his right arm, and if you don't defeat his left arm, he'll grab the brother to prevent you from using bros attacks and then fling them away to deal potentially massive damage to them, which is really annoying due to the button mashing you have to do here is especially awkward. Second, his hammer attack is extremely difficult to avoid given the fact that he targets both brothers, without you even knowing about this even if you somehow manage to counter his first hammer strike. Third, and worst of all, he can summon enemies in amounts that should be limited to dream world enemies, and while those enemies fight you, he goes to the background during the time those enemies are on the field and will spawn meat every single turn that can recover his health, to both himself and his arms (Without the Fire Flower or the Zee Egg, this will be ridiculous), granted you can use the taunt ball to bring him to stomp on these enemies, but if there's only like 4 of them left and you use the taunt ball, Bowser will just go straight back to the background, so you essentially wasted that turn. Finally, he can also summon an annoyingly persistent background enemy to constantly shoot cannonballs at you, and will take a while to take down without taunt balls.
Big Massif in the Dream World. If you are to kill any of his Mooks, depending on how many you killed, Big Massif gets a Power Bonus; since all of your Dream World attacks are Herd Hitting Attacks, this is almost guaranteed to happen. Worse yet, he summons more and more of them every turn. You can't just relegate yourself to weak attacks to avoid killing them either, as Big Massif has a chasing attack that grows in strength for every Mook he has with him, which is further boosted when he's pissed off. You have to keep the Mook count to a minimum and - more importantly - get good at dodging Big Massif's attacks, or he'll mop the floor with you.
Speaking of Big Massif, there's also his disciples. All four of them have a special gimmick in their fights that turn each of them into a Beef Gate (especially BeefCloud) and require you to have a working knowledge of all your battle skills up to that point. If you don't, then they could be even more frustrating than Big Massif himself.
The hardest of the disciples is Thunder Sass. He has the lowest count of HP for all four disciples, but this is well made up for by the brutal gimmick added to the fight: you can only counter-attack him. Yes, the Jump and Hammer Attacks are banned in this battle, as well as your Luiginary Attacks. Oh, and badge effects are banned too. This means that healing is the only thing you can do when you get your turn. So if you can't counter-attack, this battle is physically IMPOSSIBLE to win. Combine this with hard-hitting difficult-to-dodge attacks, and you've got a recipe for one of the more difficult bosses of the game.
Pi'illodium, which continues the trend of a late boss being a frustratingly difficult killer robot (kind of like Junker from the last game, who is another That One Boss). Now the boss itself is not THAT difficult to get over with, you need to break both of its wings to be able to deal more damage to it, but the attacks it uses are just painful. For example his hammer attack (see That One Attack above), where it not only goes offscreen for a few moments before striking, so you can't really tell when he will strike, but even the strike itself has such a cheap timing, it would be a miracle to properly counter this attack (seriously now, what is it with this game and hammer attacks that bosses use being so frustratingly hard to counter?!). But that's not even the worst part, the worst part is that that hammer attack has the chance to cause dizziness, which means the Pi'illodium will most probably aim for the dizzy brother with another tremendous attack afterwards. And that means you have to waste a turn to revive your dead brother, only to have your other brother probably become dizzy as well. And the less said about his missile chase attack, the better... oh, and there's also its last-resort self-destruct timer. The timer runs in real-time, meaning it doesn't stop, no matter what you do. Better know what you're doing at that part.
The third (and proper) fight with Kamek is a complete pain from start to finish. From the beginning, he splits into four clones, all with their own attacks. His attacks are easy enough to dodge, but one of his bodies buffs the others stats, and another heals for just over 100 HP (something that regular attacks can barely break when they have their defense buffed). Now remember that, since it's the Dream World, you only have one move between the enemy attacks. Combine this with how all of them can attack one after the other and you have a boss that takes ages to beat.
That One Level: Dreamy Somnom Woods can easily be this. The maze? Check. Lots of puzzles and confusing rooms? Check. That One Boss? Well, the Zeekeeper is fought at the end (with easily broken motion controls!) and Pi'illodium is fought at the beginning (with hammer attack and self destruct timer), so you end up having one annoying boss before you enter and one annoying one fought to leave. Oh, and you'd better not mess up the Zeekeeper fight on Hard mode, unless you want to see a lengthy cut scene about ten times.
That One Sidequest: The Mad Skillathon Challenges. Battle Broque Madame. The Battle Ring in general. The Giant Battle Ring even compared to the last three. And then the Hard Mode Battle Ring which is pretty much the meanest, most difficult part of the entire game due to extremely stingy turn limits and long, difficult battles with hard to dodge attacks.
Some people are already giving this game flak for having Bowser be the final boss. Though as mentioned above, the move is being received much more positively than in Paper Mario: Sticker Star, due to Bowser breaking out of his Dumb MuscleBig Bad Wannabe niche usually seen in the Mario RPG's while still keeping his personality.
Critics had hoped for the Dream World sequences to be much more surreal than they ended up being.
Considering the main plot of the game is going through Luigi's dreams, some had hoped to see more references and cameos of past villains as well as take the game down a darker road. Especially with the concept of Dream's Deep, being an especially trippy area representing Luigi's deep subconscious. After a surreal first entry, not much was really done with the idea, as it is only ever explored once again and that time was more for starting the Ultibed quest than anything with Luigi or his history.
Viewer Gender Confusion: Antasma was commonly mistaken for a female when previews for the game first came out, thanks in large part to his gender-ambiguous clothing, his feminine sounding name, and his resemblance to Cackletta, a previous female villain whose gender was more easily discernible.
Luigi as in most games fits this pretty well, especially in the earlier parts of the game. The game also has Prince Dreambert, Wiggler, and Sorrow Fist (the last of these to the point where Luigi feels sorry for him).
Woobie Species: The Pi'illos in general count for this, since they were trapped in the dream world after they stopped Antasma for the first time.