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.
The Melc Crystals. They have two forms that they can switch between at any time, each form with its own characteristics. Its first form is just like any regular old boss, albeit slightly more powerful. However, its second form, that of a bird, is an incredibly fast and powerful monster of a boss that can tear players to pieces. Did we mention that simply changing forms deals damage to everyone and can cancel?
Also, Emelious has given players some trouble. The player must not only deal with the onslaught by three opponents (the sword, the orb, and Emelious himself), but also must keep the aptly named God Slayer from reaching its acting point—if it gets the chance to do so, the entire party will be instantly killed.
The final boss is no pushover either, boasting 120,000+ HP and incredibly powerful attacks. Not only that, but it gets fiercer and fiercer the less HP it has.