I had some issues with Baby Bowser until I decided to stop using slow heavy hitters like K. Rool and Bowser and just try and hit him from a distance with Toon Link and his projectiles. The last Spirit I got was Black Shadow which took me a few tries. I guess the Spirit that overall gave me the most trouble was probably Pyra as I eventually decided to just purchase her spirit and call it a day...
Pyra
needs a Spirit that makes you immune to the Lava Floor, I tried to face it without one and... no, not happening.
I found the Eggman fight pretty difficult.
Tbh, One Piece sounds like it'd be a pretty fun adventure story about finding a mysterious treasure but good god, is it filled with padding that goes nowhere and the anime only makes it worse.
I've read on some of the powers and they're actually pretty fun and creative, even ones that are simple elemental manipulations but I'd be damned if I'm ever going to read that whole thing.
One Piece actually has very little actual padding, it's just a long story because Oda
loves world building, pretty much every arc is important in some way.
The only story arcs so far that are
actually superfluous are the Island of Rare Animals arc and the Long Ring Long Land/Davy Back Fight arc, but even the latter ended with something important.
The thing about the Davy Back Fight is that it was meant as a breather arc, since the arc immediately before and after it were huge by the standards of the time; the anime makes the arc worse by doubling it's length and then moving the actually important bit to the filler arc that comes after it, not to mention that the Davy Back Fight is preceded by one of the best Filler Arcs ever (the G8 arc).
The recent Dressrosa arc is the longest arc so far, and that one I can see complaints of padding with, right when you think the big climactic fight is going to start, the villain literally kicks Luffy out of his lair and has one of his minions create a mountain that the heroes have to climb up.