During Disc 3, find the Ultimate Weapon hovering over a lake. Defeat it and it will retreat as before. If you happen to find it hovering around the ocean, run in to it. This will make it find a new place to destroy. Defeat it each time you see it hovering over one place. The final battle with it will take place over Cosmo Canyon. After you defeat it there, it will crash to the ground. This will give you access to the Antiant Forest and Cloud's Ultimate Weapon.