Big Corpse

Defeat Giant Worm.

This is story-related and cannot be missed.

This is the boss fight at the end of chapter 5, Desert.

The Giant Worm has three tentacles, and each tentacle has two basic attacks: it will either slam down on the ground with a wide area of effect, or it will swoop down and grab Ayumi in its mouth, then slam her into the ground. The swoop/grab attack cannot be dodged, so any time the worm uses it, hold down L1 Button to rewind time and cancel it out - most times the worm will move on to the slam attack, but if it uses the swoop attack again just rewind time once more. In order to avoid the slam attack, you need to keep an eye on the tentacle that is going to attack you (only one attacks at a time) - when it starts to "wiggle" (I know - ewwww) is the time to move: hold down R2 Button in order to slide out out of the area.

When the tentacle slams down, get to it, hack at it until you have enough of a gauge to cast a spell, then hit it with a Heavy Strike; repeat.

After the first tentacle is defeated, the second will attack you and a third will throw boulders at you; also, you'll start to be attacked by Sand Cuttles (the little creatures that jump out of the sand at you); don't bother with the Cuttles unless you need to - just run around and avoid them, and keep your eyes on the tentacle that will attack you. After you've destroyed the second tentacle the third will attack, alternating its attacks with the occasional boulder toss; also the Cuttles will return.

Try not to let your health drop below half, as any of the worm's attacks can kill you at that point, always remember to time rewind out of the worm's swoop attack, and this fight should go pretty quickly.