How do I defeat Xion on Day 357?

Well, hopefully you're around level 60 or so. When you battle her, watch her attacks closely, she has patterns. Air Dash is useful to dodge some attacks (it's faster than running) so equip that with at least another level. Since most of her attacks are predictable, you can tell when, where, and how she's attacking. Glide is also useful (especially in Halloween Town when she shoots those orange things at you). If you can keep away until she's done attacking, she becomes vulnerable. It helps to have all the cures and a bunch of potions and ethers as well.

Put elixirs , hi potions and ethers/hi ethers (alot)put a limit boost and a final limit put a quad cast filled with cura's and put a magic ring and a dodge roll

For the second form... Dragonx254 gave a detailed answer for part of you problem. The third form is tricky, because she is not only much, much faster, but she has less openings, so your timing has to be quick. I honestly, though, don't use magic when I'm against Xion, except for Cure. But I would recommend to bring a lot of high potions--they would be very useful. For the third form, glide around when she attacks, and keep a good distance from her, because when she spins, she'll take up a lot of grown, very fast. When she creates the erm... Whirlwind on the ground, use air slide to quickly avoid it (it cannot be blocked for any forms) and attack afterwards. Her third attack is when she jumps high up in the air and then slashes after she lands. Air slide in the opposite direction, then quickly do damage, then glide away as she starts her next combo. This form is tricky sometimes when you keep you distance, and she heals. So when she's chasing you, move away when you notice she'll attack, but keep close, or else she'll heal, because the third form (in my opinion) is way overpowered. Finally, is the forth form, which is extremely easy. When she lowers her keeyblades diagonally, just block to avoid the attack (or else you'd have damage afflicted, and get a burn or Lv.1 defense). Before she attacks, when she slashes, just glide next to her and jump to avoid the final slash. When she uses the whirlwind, air slide, then glide away as quickly as possible from it, and if she shots the blue lasers, to her body, around her arm. If she says 'get ready, Roxas' it's her final limit, and air slide and glide quickly away from it and outside of the golden outline, or else you'll be trapped and recieve a lot of damage. Xion's easy once you find out her patterns, so good luck.

Does anyone have a good answer to how i can defeat the final one of xion's form's? I keep dying when she starts pulling out those weird lights that shoot down randomly out of sky. HELP!

Equip yourself with many hi-potions and cures and curas, also I found using fire and fira helps too in the first and second forms.