This guide is meant to be for the hardest bosses in the game. It's meant to give you the best possible strategy there is to defeating the more harder bosses in the game.

Xaldin: This is one of the first rage bosses you will meet. Unless your overleveled for this fight then he's going to prove a challenge. Lets get into his stats first.

XALDIN
----------
HP: 1155
ATTRIBUTE: Wind
WEAPON(S): Lances
DIFFICULTY: ***
RECOMMENDED WEAPON: Circle Of Life
----------
STRATEGY: This is no cake walk and you will die. But that's actually okay because you will most likely get Mickey to save you. The easiest way to get hits on Xaldin is to use the "Learn" command and stun him for a bit. Get a combo on him and deal a bit of damage. As soon as he starts to turn green you need to run away because he will be invincible. What you must to then is get an air combo on him and it will hopefully break his berserk mode. If you end up dying you will most likely get the "Don't give up" command and you'll begin to play as Mickey. What you should do is run away and get a combo on him then run away again. When your drive gauge is at least 3 bars heal Sora and bring him back and repeat the process.
----------
LOADOUT: I would recommend having "The Circle Of Life" equipped for the MP Haste ability. I wouldn't recommend using spells on Xaldin, SAVE IT FOR CURA. Magic doesn't really effect him that much either way. It's also recommended you stock up on hi-potions and put them in your quick use spot. I'd recommend equipping trinity limit, ariel recovery and guard for this boss
----------
Well that's all for Xaldin next on our list is none other than Xigbar

Xigbar: This no doubt was one of my least favourite bosses due to it's difficulty. So with great pleasure let me tell you all how to kick his a-

XIGBAR
----------
HP: 1300
ATTRIBUTE: Space
WEAPON(S): Arrowgun (Sharpshooter)
DIFFICULTY: *****
RECOMMENDED WEAPON: Sleeping Lion or Guardian Soul
----------
STRATEGY: Start off by using "warp snipe" to get Xigbar to come down. Hit him with a quick combo then back away before he begins to shoot. The most important ability to have in this fight would be guard. Use it to block all the arrows he is shooting at you and deflect them back at him. If you can master using it in this fight you've already won 40%of the battle. For the next part you have to know when to attack him with a combo. When ever he yells "Alright, reload", "Roxas" or "Sora" then he is reloading his weapon. It is the perfect time to attack him with a combo or two. One of the most annoying things about this fight is that he teleports around the battlefield quite a bit. You just have to know where he is and how to time when to guard his shots.
----------
LOADOUT: The recommended loadout for this would be nearly the same as Xaldin's. Stock up on hi-potions and save your magic for cure. The most important ability to have is guard in this. It will allow you to deflect 80% of his attacks. If you're going to use drive I would recommend going all out using either Master or Final. Do not use this early on though, use it when you have gotten little more than half his hp down. That is wen he starts to play serious.
----------

That's all I can give you for Xigbar, now for....Sephiroth.

Sephiroth: Compared to KH1 they actually dumbed him down a bit. But nonetheless he is technically the hardest boss in the game. He's just a little harder than Xigbar.

SEPHIROTH
----------
HP: 3000
ATTRIBUTE: Darkness
WEAPON: Masamune (A 7ft freaking sword)
DIFFICULTY: MAX
RECOMMENDED WEAPON: Ultima
TIP: Lots of elixers....lots
----------
STRATEGY: As soon as you begin Sephiroth will attempt to do a cut combo on you. Use the reaction command and block the attack. After that you can either do one of 2 things. You can use your limit on him and start knocking down his hp right away. Or you can start by using combos to lower his health. The easiest way to attack him is to use high jump and glide to get away from him and when your behind him get a quick combo on him. The most effective is when you successfully use an air combo on him and have combo plus and horizontal and vertical slash equipped. Once you get half his hp down he will use his most powerful and unavoidable attack. You can probably get a few hits on him before he uses it but it won't stop the attack. The attack he uses is called "Heartless Angel" and will instantly take your mp and hp down to 1. This is why you should have a lot of elixirs in your quick use. By this point in the game you should have plenty anyways. You'll have a time period to use it so use it quickly. He will also use the cut combo more than once so be prepared to use the reaction command. Another attack you should look out for is when throws meteors at you. You can use guard to block these or hit them away. Keep using cure because like any other tough boss magic isn't really effective on him compared to just hitting him. Get as many limits on him because it does a lot of damage.
----------
LOADOUT: By this point you should hopefully have the Ultima weapon if you want to beat him easy. Sticking up on elixirs is key because you will lose a lot of hp and you will need to use it after he uses "Heartless Angel". For abilities and spells you should have and use are ariel recovery, reflect, combo plus, horizontal and vertial slash equipped along with any other standard abilities. But most important have one more chance / second chance equipped. It will save your life a lot if you know how to use it. Using reflect to block and defend yourself will guaranteed make the battle 20% easier, just make sure to have it as a quick spell. If a combo is about to kill you and you are still in the air knocked down don't use ariel recovery and any damage he does cannot kill you. Once you land use and elixir or cure and keep on fighting.
----------
That's all for Sephiroth good luck on beating him.

More bosses coming soon...

Nice guide! I believe these are the toughest bosses in the original release, so I don't think you missed anything. Xaldin was tough but I passed through him, but Xigbar... Xigbar is so friggin' annoying! He takes forever to defeat, 'cause he starts shooting at you again after you hit only one combo. Nice strategy with using the guard ability... I wish I knew that, otherwise I probably wouldn't have spent a couple hours trying to beat him. That stupid anti-form screws me over every time. He goes down a lot more easily if you use Master Form, but Anti-Form is only good for when he uses his special ability (the laser storm thingy).

But with Sephiroth I have a couple very important tips you forgot.

1. The Ultima Weapon isn't required at all. Yes, it has MP Hastega which is good for using cure, but there's one weapon that's also very good. That weapon is the Decisive Pumpkin. I believe it's the same length/reach, it's easier to acquire, it has the same amount of strength, it has a magic boost of 1 (which isn't actually an advantage, but Sephiroth is immune to magic anyways), and it has the Combo Boost ability. It's perfect if you don't have the Ultima Weapon.

2. Equip the berserk ability. Horizontal Slash, like you said, is very effective. However, if you use cure and you have berserk equipped, then you can hit him with Horizontal Slash an infinite number of times until you regain your magic.

3. This one is a very important tip you missed. The Reflect ability is crucial when fighting Sephiroth. Equip it to your quick-use menu and the battle practically becomes 40% easier. Why's this? Instead of having to use Glide to dodge all his attacks, you can use Reflect for a bonus (I recommend Reflega, the highest level). A) It's more effective than trying to run away/dodge him, B) It can be used to dodge his air attacks, C) It blocks his combo move that requires a reaction command, and most important of all... D) Reflega deals massive damage if you deflect his hits, which is also a bonus for my C) reason.

TheFadedWarrior : The anti form is one of the reason I don't recommend relying on forms for the whole battle. It is good for attacking and speed and you basically can't get hit with it but it's just annoying when you can't beat the boss and his attacks are weak.

I actually meant to put reflect instead of guard in the Sephiroth battle so thanks for pointing that out.

Also I suggested Ultima instead of Decisive Pumpkin because their reaches are fairly close and Ultima has a higher attack power with is good when you are pulling off the large combos. Mp hastega is also a really useful ability because like you said reflect and cure is something you will be using quite a bit so the more you are less dependent on your elixirs the batter. But it would probably be my second choice from Ultima. But if you really want to deal some heavy damage to all the bosses I recommend "Sweet Memories" for the most devastating blows