The first thing to consider is that his openings last way longer than normal. About twice as long as a normal boss, almost 3 times as long as a Datas. He has "armor" that you chip away at with each hit, so you definitely want to keep on him, since your first combo string is going to get mostly eaten. That "armor" resets everytime he starts attacking again.

Fire and Reflect are both very good in this fight. Fire has the same amount of Revenge as a basic hit, but is stronger and multihit (for the armor).

It helps a lot to know how to handle each move:

Flying around with the Key: He usually does 3, but be ready for more. Guard or dodge roll the first two, but always dodge roll the last one -towards- him (be ready for the Keywhip though). If he lands you want to take the opening. He gets harder if you miss openings.

Glider: Same thing as the flying key, but the timing is trickier (don't get antsy and guard/dodge early), and you really, really want to get that opening. One of his moves that he can do if you miss it is very deadly (when he summons 2 lasers to hang out with him while he bikes around).

Ultima Cannon: If you are near him, get close (but not -too- close) and Guard it back at him. It'll launch him, remove his 'armor', and make him open. It also counts as roughly 1 combo string for his Revenge, so don't push him as hard.

If you are too far away, and Guarding won't return it to him, -dodge roll into it-. The i-frames will keep you from damage if you time it right, and you don't want that thing floating around while LS is doing his own thing.

When he runs at you, he'll either jump over you or do one of the next two:

Staff: As soon as he jump, dodge roll. If you get caught in the AoE without i-frames then:
-Magic Seal: Get in one combo without having to heal.
-Attack Seal: if it's still the first 2/3 of the fight, he'll be spamming Ultima Cannon, bounch that into him. Otherwise, Thunder spam while still close enough to grab the MP bubbles coming off of him.
Limit Form's Limit works on one or both of these, but I don't remember specifics.
He can also teleport straight into this move, without the running start.

Shield/Drill: Guard this twice. The second won't look necessary, but it can be. He's still open after the second Guard.

Whip: He will often follow the keyblade flying with this. He can also do it on it's own. Quick Run away. Dodge Roll also works, but gives you a wee bit less time to react. Guard can work, but I don't recommend it. You can also preempt it and hit him with a Thunder to launch him a bit so that it spins above your head and misses, but that's just showing off.

Bow: When you hear the shot fire, roll. He'll usually do three. When he puts the bow away he's open. If you are close to him while he's firing, when rolling an arrow, roll towards him and just stand in front of him. He'll fire over your head.
-If you get hit: Wait for it to slow down as much as you can, or until it feels comfortable to stop. You can move while the roulette is going.

Desperation Move: There's a bunch of ways to deal with it, but I think this is the easiest:
1st phase. Don't glide away, you don't wanna get backed into a corner (yet). Guard it. After the veritcal slam, unlock, aerial dodge away into a full speed glide in prep for the second part
2nd: Glide, but look for a wall. The rocks can work too. You want to end your glide with your back to the wall.
3rd: if your back is to the wall, you can Guard through all of this, even the last hit (might wanna roll that one though). If you don't get hit by the last hit, he open.

If you miss an opening in the last 1/3 of the fight, he can start breaking out magic. You really don't want that. But if he does, your best bet is a Limit if Goofy is up. Or a lot of Reflect if he isn't.