Hmm. A though choice. I consider both of them very hard, and they are more harder if you challenge them with Terra, since his evade ability is pretty bad. So lets analyze these bosses.
Mysterious Figure, an time traveling troll is really hard. Almost everyone of his moves kills you with one hit without second chance and once more. He has all kinds of attacks raging from a long combos to casting meteors and mega flares. Furthermore he can rewind back time, and gain back some HP during the process. You basically just spam dodge roll the whole fight.
No Heart his also really difficult, but unlike Mysterious Figure, his regular attacks won`t one shot you. Although this dosen`t really matter since most of his attacks are deadly combos with MX`s Keyblade. Combining this with his updated speed, he becomes even more dangerous. Sometimes No Heart is so fast, that I can`t react to his moves at all. He also has this attack which is similar to one of Sephiroth`s moves. He flies up to the air and casts an massive explosion. It leaves you to low HP, and puts all your commands to reload mode.
So both of these super bosses are hard, but I think for me Mysterious Figure is harder. You have to have good endurance since dodge roll spamming will numb your thumbs. Also he can take your commands away with tornado, and a single mistake can ruin your good run. Thank God in the Final Mix version you can escape his whip attack.
Was Mysterious Figure tweaked at all? Because I swear, he was so much easier to beat on the PSP version. I beat No Heart on my first try (which, I thought he was supposed to be harder than the other secret bosses?) and yet success against MF continues to elude me. I could always try to beat MF with Terra or Ven, but Aqua's playstyle is my favorite and the one I am most accustomed to.
All I know is No Heart was nowhere near as bad as MF, who either makes clones of himself, one hit KOs me with Aero, turns invisible, throws me in the air, scatters my commands, etc. All No Heart did to me was force me to attack using only Barrier Surges and the occassional shotlock (which MF will never let me use).
So i defeated both of them with aqua and this is my opinion. MF is really strong. Its impossible to predict his moves since he act casually through all the battle. His attacca are Made to kill you in one hit without OM and SC at any lvl. Dodge and attack When he does the X thing, just immediately After Aero and at the beginning of te fire move. With terra you just need to è lucky as hell to beat him. No heart is strong as well: incredibly fast his attacks are both Made to damage or to limitate your moviments. However i personally found him less stressing than MF. I was able o defeat him at lvl 52 in proud mode at first attempt. He is more regular and predictable than MF. Just use the block with good timing and counter attack.