Anticlimax Boss: The Tainted Insect, the Final Boss of the original game. Extremely high HP and Attack, and gives off an amazing amount of EXP, but it is otherwise a vanilla monster that falls to a Staff of Sloth as easily as nearly every other monster in the game. If you got to this point, you've already dealt with much worse, and the status-inducing Skull Wraiths surrounding it are far more deadly than the Insect itself. In the Updated Re-release, its queen is even more anticlimactic since she's in a room all by herself and again, has no special abilities.
Broken Base: The third game. Fans are divided over the addition of an easy mode (which, due to party mechanics, you HAVE to pick if you don't want it to be incredibly frustrating to the point where it's not even fun), splitting of the game into multiple small dungeons as opposed to one segmented one, the retainment of levels after death, and, most of all, the game's grand story, abundance of Unskippable Cutscenes and fleshing out Shiren's background. (as opposed to an Excuse Plot and Shiren being an AFGNCAAP) Some enjoyed it, while others found it quite tedious.
Demonic Spiders: The Ark Dragon. Its special ability is firing a homing blast of fire that goes through walls, does 50 damage that can only be reduced if you have a Dragon shield, and can target you from anywhere on the floor, even if you're in another room. And they can use it indefinitely. The only way to survive that is to either use healing items or Invincible herbs until you can find and kill them, or be an Inferno. Seriously, using a Scroll of Removal against them is completely justified. Only in the first game can you use this same power after eating their meat. And their higher-level form, Hell Dragon, is even worse, as it does 60 damage and is near impenetrable unless you've upgraded your weapon to near max. Good luck surviving the last half of Fay's Final Puzzle, as the Ark Dragon and the other most goddamned of the Goddamned Bats start appearing in droves.
Cranky Tanks, which can attack twice in one turn, deal 50 damage apiece, and have splash damage, which can appear when their lower-level counterparts, Popster Tanks, manage to kill something ally or enemy. Keep in mind Popster Tanks have splash damage, and deal large amounts of damage, making it very easy for a Cranky Tank to appear in moderately crowded rooms. Also, like the Hell Dragon, it is impervious to all but the most upgraded of weapons.
Many other monsters are higher level forms of Goddamned Bats. The Radishes eventually gain the ability to put you to sleep, leaving you a sitting duck unless you have a Sleepless Armband, and its max level form's effect, which turns you into an uncontrollable berserker, cannot be resisted. The Reapers learn to pass through walls while attacking twice, meaning they will home in on you from anywhere on the floor and if attacking through a wall, you cannot retaliate. The Skull Mages' effects turn from annoying to debilitating when they become third-level Wraiths (including ability sealing, Level Drain turning you into a riceball, etc.), and become even worse as appropriately-named Skull Demons at Level 4 (who can drain levels 3 at a time, put you to sleep, and even blind you).
But what truly stands out about the game's Demonic Spiders is that several monsters are savvy enough to attack your inventory. That Shield you melded with multiple buffs, and would otherwise be a reliable defense in most other RPGs? A high-level multiplying Slime can dissolve those buffs away one by one, or an Armor monster can fling it away into another monster. That Herb of Revival you keep out to guard against an unlucky situation? Even the lowest-level Fowl can zap it into nothing, or a Gaze can hypnotize you into eating it. Kept a bunch of items in a Jar? Hope that Item Bandit doesn't turn it into a useless Weed. The Gazes themselves deserve special mention as they can render you helpless for turns on end while other monsters slowly whittle you away.
Game-Breaker: Bufu's Cleaver in the DS rerelease of the first gamenote Since by the time you get it in the original Super Famicom game, you have nothing else to use it on, while the DS gives you three extra dungeons that permit the Cleaver. It randomly drops meat from any monster you defeat, said meat allowing you to transform into said monster. While you need to go through a difficult reset dungeon (Kitchen God Shrine) to get it, it also teaches you just how busted it can potentially be when you take it out of the dungeon and meld it to a stronger weapon. Some of the monsters' abilities guarantee virtual invincibility under player control, especially since they are unaffected by traps. For instance, the Death Angel / Death Master can move and attack twice as fast while passing through walls, and monsters cannot fight back if you attack them while in a wall tile unless they can also pass through walls. Even the weaker Pacorepkin family can be broken just because it has the last ability as well. Even better, the Air Devil family makes you invisible, which means monsters will never attack you while you can slaughter them to your heart's content. The disadvantage of meat is that you can't use other items until you revert, but considering you already have a free pass to the stairs for one floor, this can be moot with the right monster. Because of this, it was nerfed in the third game, making it a breakable weapon.
Goddamned Bats: Where to start? We have undead Radishes that throw status-inflicting herbs at you, those annoying archers/tanks that move away from you while firing projectiles, reapers that move twice as fast and hit hard, skeleton mages that throw all sorts of random effects at you...and at higher levels, many of them become outright Demonic Spiders as mentioned above.
Hilarious in Hindsight: One of the most powerful items in the game is a Blank Scroll, on which you can write the name of any scroll you've previously read or, under certain conditions, gain access to two other powerful effects. In Kung Fu Panda, the Dragon Scroll is said to provide unlimited power to whomever reads it, and, it turns out to be a Blank Scroll showing the reflection of the one who reads it.