Ok, here are BlazeHeatnix's tips for Critical mode.
First, don't choose the sword. Ever. Not unless you're a KH Expert. Second, I recommend not bothering with Retaliating Slash, Dodge Slash, Slapshot or Aerial Sweep. They just get in the way. Don't equip any Combo Plus's or Air Combo Plus's either, unless you're fighting Xaldin and Xigbar. And if you practice enough, you won't need Magic Lock-On either as most of your offensive magic has a high enough AOE that you usually won't need it. Finally don't equip any "Auto" abilities. Just don't. They're hazardous to your health. Learn to get through your menu quickly.
You have magic. USE IT. Limits, summons, drives, all will help you tremendously. You can't get through the game mashing the X button with the occasional heal anymore. Use Chicken Little when dealing with a lot of mobs, use Genie and Peter Pan for bosses, and use Stitch for the coliseum. Firaga and Thundaga will absolutely WIPE THE FLOOR with mobs and some bosses, don't underestimate them.
Do not equip every single new Keyblade you come across. Unlike KH1 where pretty much every new Keyblade is better than the last, here different Keyblades are better for specific situations while some are just generally good to have. My pattern for the main story is Kingdom Key - Hero's Crest - Circle of Life - Oathkeeper - Rumbling Rose - Decisive Pumpkin. The Ultima Weapon sadly just isn't as good as the Decisive Pumpkin, in any situation. I know it's cool, and feels good to hit stuff with, and it's the friggin "ULTIMA WEAPON" but it's factually an inferior weapon, plain and simple. For TWTNW and Cavern of Remembrance, you may want to swap back to Rose, as it's better suited for heavy mobs than the Pumpkin. When fighting Demyx and Xaldin, use Bond of Flame and Guardian Soul respectively. For the Drives, change Valor form from Star Seeker to Oathkeeper once you get RR, and slap Hero's Crest and Oblivion on Master and Final form respectively.
For Donald and Goofy, as far as customizing them you can leave it on default, and you may not care that much about their armor and accessories as no matter what they're equipped with, they'll die pretty quickly in any major boss battle. But you'll still want to keep them alive, because they serve two important roles now: Limits, and Drives. Donald's Limits are great for mobs, and Goofy's limits are great for bosses.
---------
Now, for Zexion, here's how I beat him, and I did it after just clearing Timeless River. Once the battle starts you'll want to alternate between destroying the books and wailing on Zexion. Freeing your friends should be your top priority, and using fire helps. You'll also want to use Upper Slash on him to get some air combos in. Once you eventually end up in his "domain", go into Valor form and just wail on him. Once Valor form is all used up, even if your drive gauge comes back, I don't recommend using it again.
From now on once you enter his domain, cast Reflect a few times to avoid getting hit by the book's "flash" attack. If your menu gets messed up, repeatedly pause the game with each page turn to check the options. If you don't see anything that might mean "heal" or "release", turn another page and repeat. If you don't see the red book when you immediately enter, look around and find a book that's seems to be controlling the rest. That's Zexion.
Now, the meteors. This is pure luck and tricky to get into the right one fast without Valor form's speed or Dodge Roll. If you don't make it, quickly keep casting Reflect to avoid the meteors or else you're dead. After that, just keep attacking Zexion whenever you can, and don't bother freeing your friends anymore.
