-Obtain and level up all drives and summons (Great Guide Here)
-Complete Puzzle Mode to obtain the Grand Ribbon (Again, lots of guides)
-Synth 2 - 3 Ribbons (Petite Ribbon + Serenity item)
-Obtain/synth good accessories (The Cosmic Arts can be found and is amazing. The Moon Amulet/Fencer's Earing/Mythril Ring are also good).
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-Obtain the Decisive Pumpkin and Bond of Flame Keyblades (both are obtained by the time you get to The World That Never Was)
-Level up until you obtain Once More, Second Chance, Finishing Plus and Leaf Bracer.
For Keyblades, you're going to want to use the Decisive Pumpkin. It's going to deal the most damage in most circumstances.
For Armor, equip the Grand Ribbon and as many Ribbons as you can. They not only offer resistance to all elements, but also resistance to basic attribute damage (it's an unmentioned effect). The only fight where you don't want to do this is Larxene. For her, you just want to stack Thunder Resistance. Other than her though, go with all Ribbons.
For Accessories, use the Cosmic Arts and Moon Amulet/Fencer's Earing/Gold Ring. Whatever you have/can make. Be sure to stack strength and magic. AP doesn't matter much.
For Items, I used basic ethers and Hi-Potions. They can be bought pretty easily after some money grinding. You'll have plenty of money to buy them after leveling Master Form.
I'd personally rank the difficulty of the AS fights as such:
Marluxia Zexion -> Vexen -> Marluxia -> Larxene
As to how you're going to beat them?: Final Form Firaga and Stitch.
It's really that easy. Use Stitch for Lexaeus and Larxene, Final Form Firaga for Vexen and Zexion, and Marluxia you have to deal with on your own.
Larxene: Reflect her first charge and summon stitch. Use him to reflect all of her attacks. This fight can get pretty annoying, but stick it out.
Lexaeus: Same thing here. Summon Stitch while he's charging up and use reflect on all of his attacks. When you stun him go in for full combos.
Vexen: Break his shield by starting the fight with a slide dash into a reflect and then a guard break. Go Final Form, spam the first two hits of Final Form Fire (Just the first two hits, NO FINISHERS) and watch his HP Bar Melt. Make sure to equip Bond of Flame as either your main or Final Form Keyblade.
Zexion: Same thing. Use Firaga to break Donald/Goofy out of the book, go Final Form, murder Zexion with Final Form Firaga. I actually laughed when I first did this fight because of how fast he died.
Marluxia: There's no way to cheese this fight. That being said, he can easily be looped. Look up videos online about how to do it. It may look hard, but you can do it with practice. Always go for Aerial Strike over Restore Count when prompted to perform an RC. Equip at least 2 Combo Plus abilities for extra reflects during his DM (the one where he looms over you and columns shoot out of the ground).
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I'd suggest watching Level 1 Battles against the Absent Silhouettes as those strategies translate extremely well to higher levels.