Remember the Mario &
Luigi giant boss battles I mentioned?
Not exact matches
Add some mini games and challenges that really do require high precision and the fact
Giant Luigi bosses seemingly can't be skipped and don't take your stats into account whatsoever, and you've got a game that can easily cause hours of difficulty hell past the dream mountain area.
The former is basically the Gauntlet from Inside Story (complete with a
giant Luigi battle
boss rush), the latter a bunch of Bros item challenges that put your reflexes to the test.
In fact, that
giant Luigi volcano
boss is pretty much the best marker of where the difficulty goes up and generally stays there, so you'd better be prepared to practice and put the effort in.