They're not really secret bosses. Vanitas in particular, is basically his regular battle, but with ridiculously high attack power. His actual attacks aren't anything special at all. Same kind of thing with the Mysterious Figure, but less so. He has lots of really cool and tricky attacks, but again wouldn't be all that difficult if each attack didn't total your health bar.
Bosses like Xemnas and Terra's Lingering Spirit had high power to, no doubt, but they also had lots of tricky and hard to dodge attacks that could be utilized to counter attack.
Altogether, to me, Vanitas' Spirit and The Mysterious Figure to me, feel like regular bosses with their power amped up.
They're not really secret bosses. Vanitas in particular, is basically his regular battle, but with ridiculously high attack power. His actual attacks aren't anything special at all. Same kind of thing with the Mysterious Figure, but less so. He has lots of really cool and tricky attacks, but again wouldn't be all that difficult if each attack didn't total your health bar.
Bosses like Xemnas and Terra's Lingering Spirit had high power to, no doubt, but they also had lots of tricky and hard to dodge attacks that could be utilized to counter attack.
Altogether, to me, Vanitas' Spirit and The Mysterious Figure to me, feel like regular bosses with their power amped up.