I'm gonna just go ahead and say as far as Secret bosses go Terra's Lingering Sentiment is probably the hardest.
He's the only only secret boss in the series that even if you can loop him and beat him no damage style, he isn't hard because of the game mechanics screwing you over. If you mess up it's pretty much your own fault for doing so, since all of his attacks are entirely dodge/guardable. That and he has 13 bars of HP and the lower you get his HP to the more aggressive he becomes.
Mysterious Figure on the other hand isn't difficulty, in my opinion.
He can be easily defeated with Aqua and Ventus; even though Terra dodges slower his power makes up for it.
If you can attack him non-stop he barely has time to be offensive that and being able to attack him while he's invisible makes a difference. On the flip side of that he's only difficult because of: clip damage, i-frame lag, random lock off and weird camera angles
Vanitas Sentiment isn't difficult, he's just got the same problems as MF's battle (it's just not as bad as that fight since you're in a wider area for fighting).
He can easily be taken out with Tornado w/ Ventus, Mine spam from Aqua and the same could be said for Terra, even though Demolition Finisher helps as well. Hell the strike raid glitch makes it even easier.
Kurt Zisa isn't difficult until he starts spinning and even then you've just got to time your dodge rolls and jumps almost exactly right, also most of his other attacks when he isn't using his orb shield are completely guardable; everything else is dodge rollable.
Phantom is just a long as balls boss battle but as long as Goofy's got MP Gift equipped, they're all equipped with Ethers and you've got Elixers/Ethers equipped on yourself that coupled with a keyblade with Decent Magic stats (Diamond Dust for FM and Spellbinder/Lionheart/Lady Luck for NA KH). Couple that with the fact that you can evade all of his swipes just by flying up and down, his DM with the death beam can also be completely avoided.
Sephiroth in KH's pattern is pretty easy of a 4 hit combo plus dodge roll to avoid his teleporting slash and if you really want to get it over with quickly you can just spam STrike Raid over and over to end the battle quickly. His KHFM counterpart however, can pretty much only be dealt with by spamming Strike Raid seeing as he spams Fire wall immediately after you combo him for any amount of hits. If you've got an insurmountable amount of HP and spam curaga/aeroga constantly then it probably won't matter for you.
Ice Titan's only difficult parts are his beginning and last phases of his boss battle, not so much hard as annoying since Sora doesn't show up in the camera when you're trying to get away from him. Leaf Veil makes the boss a lot easier though at lower levels. There's just a lot going on which sometimes can make it difficult to concentrate but he's not that bad.
Xemnas (last but not least in KHFM) He's not hard because he's easily looped with Graviga/Slide Dash/Zantetsuken/Gravity Break combo.
The only reason he becomes difficult and this goes back to how MF and VS are annoying; the game mechanics make him difficult not he himself.
Aside from his capture ability and sometimes his final Orb DM, Donald and Goofy make this fight annoying. If they are alive and moving around close to him it makes him switch up his pattern, but take them out of the equation and he's not difficult at all (which is why killing them at the start should be your best bet).
Might I mention that Xemnas and Kurt Zisa from KH/KHFM allow you to summon Tinkerbell to help if you're really having trouble with the boss battles.
Couple all that information and Terra is the most difficult thus far out of all the secret bosses thus far. Personally he's not that difficult for me, but just going by how the average player gauges these battles, he'd give you the most problems.
