I remember breezing through Dawn of Sorrow & Portrait of Ruin (aside from a few sections that gave me problems), but Order of Ecclesia kind of kicked my ass the first few times. They actually ramped up the difficulty which I wasn’t expecting. There was a certain attention to detail that wasn’t present in the previous DS entries.
Mirror of Fate, however, that game didn’t offer a challenge to me at all. And it has 0% replay value. No extra modes, no secrets, no boss rush, nothing. I never understood why.