Ultimate Weapon can be found floating over a small crater east of Junon.

If you’ve waited until you’ve reached the Northern Cave you should have no trouble with this boss. Keep in mind that you have to fight from the Highwind, so anyone who isn’t using a ranged attack won’t be able to use standard attacks. Use a Long Range on someone to negate this.

Ultimate Weapon will run away during the fight and will begin to fly away. Try to hit him again to re-engage as soon as you can. If you lose him you’ll have to search the world to find him flying around again. Don’t be afraid to enter the Highwind and talk to the guy in the operations room to refill your HP/MP. You won’t lose Ultimate Weapon while you rest up in between battles. The final battle with Ultimate Weapon takes place near Cosmo Canyon.