I think Mysterious Figure is harder than No Heart, and it’s really not even close. I just finished playing them both about an hour ago. Let me explain my reasons.
1) “Mysterious Figure” attacks far more constantly and his attacks almost always bring you to 1 HP. So I was having to constantly roll, except when attacking. Whenever I let up even a split second I was punished severely. On “No Heart”, if you’re on level 55 or above you can pretty much hit him at will.
2) “No Heart” can be beaten with the Mine Square move, where you just wait until he does his first set of lasers from the ground, then put down mines. In fact even if he’s gonna hit you, just do mine squares and then use Cure if he hits you. No big deal. YOU CANNOT MINE SQUARE, SHOT LOCK, OR CHEESE “MYSTERIOUS FIGURE” AT ALL.
3) “Mysterious Figure” has the move where he lasso’s you and you have 4 seconds to get out of it, or you die. And if he lassos you again (which he almost always does) now you only have 3 seconds to get out of it, which is almost impossible (I fail like 90% of the time at that). if he gets you a third time, it’s only 2 seconds.
4) You can only hit “Mysterious Figure” on a couple split-second instances in between his constant fast paced attacks, using “Thunder Surge” which is best. And if you time it wrong- or just get unlucky- he will hit you after you do it. He may even do the 1-hit-kill lasso with a timer.
Whereas, with No Heart, you can safely put down mine squares or even Keyblade attack him normally, without worrying about being killed (as long as you’re not awesome and you keep tapping “cure” if he starts hitting you).
5) No Heart’s “one hit move” still doesn’t kill you... it just brings you to 1 HP and makes your stuff reload. SO FREAKING WHAT? You just have to roll until your “Cure” reloads, then use it in between his attacks. Also, “No Heart’s” move can be dodged by simply rolling and it’s SUPER OBVIOUS WHEN HE IS GONNA DO IT... he just goes up into the sky to do the spell. So just roll invincibly and you’ll be fine.
overall, The reason mysterious figure is harder than no heart is because on “no heart”, it’s easy to prevent your death and to beat him. Simple as that. As long as you’re patient and roll anytime except when using mine square or a well timed block or hit, you’ll be fine. On mysterious figure, you ALMOST ALWAYS will be killed even if you do EVERYTHING RIGHT, because he’ll get you with the lasso or he’ll hit you into the air after bringing you to 1 HP making it almost impossible to avoid the “new combo” hit which will kill you.