There are thirteen challenging boss fights in the Final Mix version of Kingdom Hearts 2. In order to reach them you need to level up all of your drive forms in order to traverse the Cavern of Remembrance, accessible through the Postern of Radiant Garden/Hollow Bastion. This guide is not yet complete, so please contribute!
Xigbar: Xigbar's not a hard fight, but sometimes he can become a bit tough. This fight plays out almost identical to the story fight, but now he'll sometimes change the arena. Dealing a finishing hit will set the arena back. Use Air Dodge, obtained by leveling up Master Form, to deflect his bullets as you approach him. This boss isn't hard, and is a good place to grind Defense Ups. You'll want Air Dodge and Block for this in order to deal with his bullets and some of his abilities.
Xemnas: Xemnas is a challenging boss fight, mostly because it can be hard to deal with a couple of his phase one attacks. If you block and dodge with good timing you'll be able to avoid getting shredded by his multi-strike ability, especially the version that darkens the area. You'll want to go into the battle with Glide, Block, Dodge Roll, and plenty of time. After killing him you go to phase 2, where you gain Riku as an ally. This plays out almost identically to the fight at the end of the story, but a couple abilities make it even harder to hit him. If you learn how to work around his BS this can be a good place to grind Attack Ups.
Roxas: Roxas isn't a tough battle, but it's certainly not easy. With more health and playing more aggressively Roxas can be a challenge to someone who's not learned to block or dodge yet, especially now that he rarely uses his attack that allows you to steal his keyblades. With some endurance and skill you can defeat him however. He doesn't have many if any new attacks, so if you've learned to ace the story version of the fight then you won't have a problem. This can be a good place to grind Magic Ups.
Demyx: Demyx is a hefty challenge if you're not prepared. It's advisable to go in with Negative Combo and Fenrir to skip straight to finishers, then jump and attack to use Magnet Burst for the first section. This will allow you to easily wipe out the water forms that would normally prove to be an incredible challenge. Afterwards, the fight goes on mostly like normal, with a small form onslaught in the middle. This one's easier than the first and won't take much. Once you get him to low health, he will go into the final onslaught of forms. This one requires a strategy. A good one is to start off jumping and using magnet burst, then when the quantity of forms gets to be over 50 switch to Wisdom or Final Form (with Oathkeeper in Final) and spam Firaga. The AoE on the Firaga finisher will take out the forms in mass. Final Form has a bigger range, but Wisdom Form will last longer. After this it should be pretty easy to get the kill.