I swear to god if Vexen has that stupid laugh i'm gonna kick his ass more...though he shouldn't be that much harder seeing as you can run around in more area, and the card breaking aspect especially.
Larxene is fast, but just dodge rolling out of the way of her strikes shouldn't be too difficult.
Leaxeus shouldn't be hard, he'll just be fun; especially going into dark mode against him...can anyone say "Dark Aura"
Riku4, i'm not gonna lie owned my ass in that game, but now that i know exactly how to beat him he shouldn't be too hard, the only thing i really need to watch out for is dark firaga, because it homes in on you fast and it's hard to find your card before hand....dark aura well simply put he flies up before he uses it so it shouldn't be to hard to counter that...
Zexion shouldn't be to hard, cause like they do on the video just running around in circles and use sleights on him, like dark break & dark aura, hell that 3 combo stun move works wonders too
Marluxia is just a fool, but he might be really difficult in re:COM, i mean he moves really fast..a little more so than Larxene, and that flower rain combo is sometimes hard to avoid, then again more room to run around in so it should be easier to avoid
Axel, is fast as hell in COM, and if they keep him having the same style, he might very well be the hardest..although his flame wall is easy to avoid, the rest of his attacks come really fast and are hard to avoid, not to mention the fact he can get up on you before you have time to react.
Marluxia's final form is just bulky and thats it, constantly card breaking is all you really need.
as for Xehonart's heartless he was really difficult in COM, but if he's anything like the gba version, all you'll need is that Leaxeus card and the Malificent cards to help more in the battle, and again more room to run so he won't be to difficult...and one more thing in this boss battle there really is only 1 attack that will work wonders for you and thats the 3 hit combo stun...hit him with it and you'll get more opportunities to beat his ass especially from above with that air slash combo...