Am I noob (probably) or are they hell on earth? I'm talking about Vanitas Remnant, No Heart and Mysterious Figure. I won't talk about Armor of Eraqus since I beated him on my first try.

First things first, I am using Terra. The slowest, the least agile, I hate him.

But, he's on level 99. Probably high level doesn't matter.

So, let's start.

Vanitas Remnant: I had my first try on level 66. He kills you in two hits. He's fast, he teleports, causes blind and caos (dangit) and he has only one life bar, which makes me rage a lot. Plus, he heals if he sees you using Cure. Took me a hella lot of tries to beat him, and a lot of aerial slams too.

No Heart: Oh, God. What a d*ck. He can control you're movement speed, he can trap you, he's always shooting dark orbs and keyblades.

He plays with gravity and he explodes causing you to lose all your HP and commands charge. Moreover, he slams the ground causing Caos if he hits you. Need a lot of Cure and Ars Solum. Ultima Cannon very useful too.

Mysterious Figure: I only tried four times since I unlocked the fight with him yesterday, but, oh boy. He's crazy. It's pretty easy to deal damage with Surges, but if he starts a combo, you're dead. He uses tornado to knock away you're commands, he makes copies of himself, he vanishes (Crap) and I have to beat him yet.

Will they be easy with Ventus and Aqua? Level 99 Terra is somewhat useless...

P.S. Lingering Sentiment of Vanitas is, for me, way harder than the hardest secret boss of KH2FM, Lingering Will. And he's the easiest among the secret bosses here. Be ready if you want to face them, I almost cried after beating Vanitas' Remnant.

The BBS secret bosses are all pretty poorly designed. Focused more on spamming dodge and exploiting specific commands

You are starting to make me feel scared, dude. I didn't tried BBS extras yet, I'm finishing KH2 first.

 

The only think I know for sure, is that BBS battle system is broken. For example, all you need is spam some command and dash/dodge-roll/wtv and you easily kill the boss. But still, you are making me feel scared because I'm not feeling prepared for these bitches. LOL

Don't have any type of cure magic in your command deck when you are facing Vanitas Remnant because he will heal himself as well you can use potions and such that's fine.  There is also a cheap way to beat him and that's to have all your attack commands be raid moves and try to make it so a rock is between you two and just use those raid moves they will hit him it may take a while for that to work but eventually it will, unless they chanced that in 2.5 which I highly doubt.  I have never faced No Heart before so can't really help you there but I think Everglow has a video about his moves and a helpful way to beat him kinda like what he did with Lingering Will so that might be helpful.  Mysterious Figure, yeah I only beat him once as Ventus and that was after probably over 100 attempts with just him just have Thunder Surge and whatever you do make sure you never go past the first Command Style because that will screw you over big time, you probably won't even get into the first command style but still just to make sure, and the key point in that fight is just dodge, dodge, and dodge, but that was me facing him in the original BbS and I can see that you can actually get out of that Whip attack that he dose so the fight might be a little easier but most likely not.  Anyway good luck.

-snip-

raid glitch still works in 2.5 did it myself