Cloud's Ultimate Weapon Guide
In order to get Cloud's Ultimate Weapon, the Ultimate Weapon, you will need to defeat Ultimate Weapon. What a mess! You can find Ultimate Weapon (the monster) outside of Junon sitting inside of a large crater that was formed around the end of Disc 2.
To engage him in combat ram into him using the Highwind. Much like during the Mideel battle after you lower his HP to a certain amount he'll take off and the battle will end. You will want to follow him around the World Map and crash into him a few times (5 or so times usually works) then follow him until he stops and starts casting (like shown in the screen shot below).
Note: It is strongly advised that you do not lose sight of Ultimate Weapon during this quest. He flies around the entire World Map and finding him again while he is flying around is a royal pain in the ass and could take you literal hours. Save your game before attempting this quest and don't save over your main file again until you complete it.
When Ultimate Weapon stops you'll be able to fly into him an engage him in battle again. Below are a list of all the locations that you can find Ultimate Weapon stopping at (incase you lose track of him). He only stops at one of these locations if you collide with him in the air enough times. If you fly around to each of these areas and don't see him it's probably because you have to collide with him still in midair.
The Northern Crater
How many fights you have to participate in against the Ultimate Weapon until he goes to his final resting place is undetermined. He has a total of 100,000 HP and when you lower him below 20,000HP he'll go to the last place he stops, near Cosmo Canyon/Ancient Forest. This location is also pictured in the screenshots below.
**Important Enemy Skill Tip** Ultimate Weapon will use the Enemy Skill Shadow Flare as his dying move. It's a single target ability so only one of your characters will be able to learn it at this time but it's still useful to grab now.
Defeating Ultimate Weapon will create a crater in the ground (shown in the screenshot above) which will grant you access to the Ancient Forest dungeon without needing a Green, Black or Gold Chocobo. Additionally, obtaining the Ultimate Weapon allows you to begin The Kalm Traveler quest and spawns Ruby Weapon.