Mirage Mania from the first game certainly qualifies as well.
Breather Level: Several levels in Book 2, mainly because the final level of Book 1 was much harder.
Even Better Sequel: Volume 2 introduces companions & rainbow tiles, features a more compelling plot, improves the Tome of Knowledge, has fewer Awesome, but Impractical treasures, is more flexible with bonus words, lets you choose which minigame you want to play & allows you to replay specific books for points. (Clips & Giggles was removed, unfortunately.)
Goddamned Bats: If you have really good spelling, everything but the bosses is this.
Goddamned Boss: The Eternal Wanderer and Frankenstein. The Eternal Wanderer's level even has several Goddamned Bats. Fittingly, the title to the level is called Highway to Heck.
Lady Summer-Speak in Volume 2.
Nightmare Fuel: A few of the bosses can qualify as this. Surprising that a worm can destroy them with the English language...
That One Boss: Pretty much every boss that has immunity to words that are particularly short (unless you normally spell longer words).
The Hydra is the first survival battle in the game, and it is capable of inflicting every status ailment you've seen in the game up to that point. Thankfully, each of the heads do not have much HP.
The fight against Codex is a battle for survival where you have to face several monsters you've faced before (which while weakened still retain their moves from before, meaning that you'll experience virtually everything you've seen before thrown against you) with Professor Codex at the end. Codex himself is capable of using pretty much every status ailment in the game against you, and he has three-AND-four letter immunity.
Even for a final boss, The Machine in the second game is awfully difficult. Its Plot Hole attack does a large amount of damage, and to make matters worse it comes with a status ailment that prevents Lex from moving for a certain period of time. And it has three forms that you have to take down in succession without healing whatsoever (although healing potions and gems can still be used).
That One Level: The Labyrinth of Crete has monsters that enjoy stacking poison against you, and to make matters worse some of them are capable of stunning as well.
Genies, Genies, Genies has several monsters that can inflict all kinds of status ailments on you, and to make matters worse each of them has armor.
The Haunt has Demonic Spiders with each of them (except for one, which is still a pain to fight) capable of powering you down, powering up, and healing themselves all at the same time. The boss of the stage even has an extra power down move. Thankfully, each of them are weak to metal words, which gives them a severe weakness to the Hand of Hercules.
The Celestial Realm from Volume 2. Stuns and tile attacks abound.
Also from Volume 2, Virtually a World. Your gems & potions are in grave danger.
The Sphinx from Volume 1 in Book 2. Rather then being a regular fight, you've got to answer riddles. This can easily become a Luck-Based Mission on whether or not the proper letters will appear.