Ultimate Weapon is one of the “weapon” bosses that come out of the Northern Crater after Sephiroth receives the Black Materia from Cloud. Your first encounter with Ultimate Weapon is in Mideel when the team is attempting to rescue Cloud. It flies away before can finish it off…
The terminology can get a bit confusing due in part to everything being named the same thing. Each character has an Ultimate Weapon (click on the link for more information about each); Cloud’s Ultimate Weapon is called 
Ultima Weapon and you receive it for defeating a boss named Ultimate Weapon (described below).
You can find and defeat Ultimate Weapon anytime after you defeat Diamond Weapon during its attack on the city of Midgar but it is best to leave this battle until later on in the game as it becomes much easier.
Ultimate Weapon is first located in a huge crater just to the east of Junon. You will have to battle it several times as it will flee from battle a few times just like it did in Mideel.
After your first battle it will start flying randomly around the world map darting back and forth, though it respawns at this crater each time it disappears from your sight, so if you can fly back to this area quickly enough you can initiate the next battle much more easily.
The actual battle against Ultimate Weapon is fairly straightforward. It does have a few powerful attacks so keep your party healed up throughout the fight. Make sure to equip 
Long Range Materia on your characters as some of the battles will be fought from the deck of the Highwind which will prevent melee characters from using regular attacks.
It has approximately 100,000 HP however it does not heal up after each fight, including the fight at Mideel. Focus on using your hardest hitting spells, abilities and summons in order to do as much damage to it as you can before it flies away.
Find Ultimate Weapon flying around the world again and bump into it a few times until it flies directly to its next destination which will either be:
Slam into it one more time once it arrives at its destination to begin the next battle. It can actually be fairly difficult to track down Ultimate Weapon at times and unfortunately there are no shortcuts to finding it where ever it has flown on the world map.
The final battle occurs near Cosmo Canyon and Ultimate Weapon will head straight there once it falls below 20,000 HP. It will not fly away from this fight.
Be careful because Ultimate Weapon will use Shadow Flare as its last attack before it dies on one of your characters. It casts it on the character who lands the killing blow and the Shadow Flare skill can be picked up and learned through your 
Enemy Skill Materia. The reward for defeating Ultimate Weapon is Cloud’s Ultimate Weapon called Ultima Weapon.
There will be a huge crater left over after the battle has concluded. This new crater provides you with a pathway to reach the Ancient Forest. Check out the Ancient Forest side quest page using the link below for more information:
