These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anticlimax Boss: Spirit grenades will kill the Dark Lord in seconds flat, but he's still easy if you don't have any. If you completely destroy his shield before hitting him (which is laughably easy if you don't let the pieces fall on you), one of his attacks does nothing, another then takes no effort to dodge, and shooting him constantly will kill him before he can kill you with the rest.
Breather Boss: Krakomedusae (the giant jellyfish), due to the fact that there's a pipe that dispenses an infinite amount of spirit stones. If you have Spirit of Peace equipped, you can easily restore both your health and mana just by sitting on it. The boss also moves in very predictable patterns, so it's not hard to avoid.
Complacent Gaming Syndrome: The Spirit of Peace is far more useful than the other badges, de facto making your health bar twice as long.
Defeating the boss in the mines gives you double damage, the ability to shoot underwater, and on your way there you're getting a shotgun-poweredDouble Jump. By comparison, defeating the boss in the Sunken City gives you the ability to breathe underwater (which at that point is more of an Anti-Frustration Feature) and a health bar extension, while the boss of the tombs gives you nothing but a MacGuffin. They can be defeated in any order, but starting with the first one makes things much easier.
The Judge, due to all the Eye Scream involved in fighting him.
That One Boss: While much of the game is Nintendo Hard, most bosses are relatively easy once you've fought them a few times, and the final boss of the good and bad endings is even easier. And then there's the Judge, who is a Goddamned Boss to Metroidvania veterans (because you have to beat all three of his forms and climb most of the way out of the Abyss before you can save) and this to everyone else. His first form is as close to Bullet Hell as any platformer should dare to get, and getting hit at all by the other two will usually either knock you into a Bottomless Pit or kill you outright.