Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
This is it. The final string of battles. If Oliver and his friends can pull through here, they will save the world as they know it. No slouching, you lot—you can prevail!
The White Witch
Your first battle is against the White Witch's normal form. She's strong, but not much stronger than your average boss. She spends most of the battle casting spells, alternating between Star Bolt (damage to one character), Pillars of Darkness (attack to all) and Fallen Star (a great deal more damage to all). She's resistant to physical attacks, so focus most of your energy on using magic to blast through her HP. If you can beat Gallus, you can beat the White Witch.
One battle down—but the White Witch isn't done yet. She transforms into something far more beastly . . . and now it goes by its original name.
You begin this battle in the same condition as you ended the last, so take a moment to heal before you go on the offensive. This form of the White Witch is a physical bruiser, and can quickly cross the field to dish out a lot of damage on your team. She has several named attacks, most of them similar to one another, though the main one to watch for is Annihilation. It hits really hard, and because it takes a while to execute you may run through your Defend before it goes off. Wait until the Witch jumps back and gets ready before Defending. Other than that, standard tactics apply—beat her down with your strongest attacks. She uses Annihilation a ton, and it's the move most likely to generate golden glims, so expect a lot of support in this fight.
You may have beaten the White Witch, but the fight's not over yet. There's still one more to go—against the Zodiarchy. They won't let you wrest control of their world without a fight.
Defend as soon as the battle begins, as Zodiarch will immediately use the all-hitting move Chaos to smash everyone. Once you've healed up you'll get a lay of the land: Zodiarch is accompanied by four Satellites, each with fairly beefy HP, all of them usually after the same target. You need to take out the Satellites, then focus your efforts on Zodiarch. The Satellites themselves aren't that tough, though if all four gang up on one character they can sap their target's HP in a hurry. They'll come back periodically throughout the battle, but killing them is still worth the effort.
Zodiarch is your primary target. Aside from an extremely painful physical attack that discourages melee strikes when it's not stunned, Zodiarch employs a few different special attacks. Chaos is painful and all-hitting; Shockblast paralyzes characters in a certain area; Sigh of Sorrow puts them to sleep. You can run to avoid the latter two, but you must Defend against Chaos or prepare to take a lot of damage. Once the Zodiarch reaches roughly half of its HP it will give up on the Satellites and employ its magic much more forcefully, so be prepared to use up a lot of those really powerful items you've been squirreling away.
(Fortunately, you have some help in this fight. A fourth character? One you don't really have to heal? Yes please!)
Trounce the Zodiarch and you'll have beaten the main quest. Enjoy the ending of Ni no Kuni . . .
. . . receive the Nix and Ashes of Resurrection spells . . .
. . . then gear up for some post-game content!