On disc 3, you can chase Ultimate Weapon to get Cloud's Ultimate Weapon; Ultima Weapon.
The first location is the Junon Crater; it is just hovering in the middle of the lake, fly into it to start the battle.
Every time you deal significant damage, it will escape and fly to a new location, each time, you must follow it and fly into it a few times to make it go to the next location.
The locations below are where it can arrive at and what kind of battle it will be:
Ground Battle:
Sky Battle:
Once you have whittled down its HP enough, it will be at Cosmo Canyon for the final battle. Defeating it will cause a crater to appear at this location which will allow access to the Ancient Forest.
Each time Weapon escapes, its HP does not recover, so just keep going at it. If Weapon is not there when it escapes, check back at Junon Crater as it may have gone back there.
Tip: If you have Knights of the Round, it is possible to make it go straight to Cosmo Canyon as this will deal a massive amount of damage.
Boss will escape and move to a new area which will either be a ground or aerial battle. Boss becomes immune to Earth and Water while flying.
Thunderball is fire based; Fire Ring or Fire linked with Elemental on Armour will help deal with this.
Ultima Beam deals decent damage to all party members.
Shadow Flare is used as a final attack and deals huge damage. It is also an Enemy Skill that can be learned if Enemy Skill Materia is equipped.
When flying, Boss can only be hit with Magic, Summons, Limit Breaks or Long-Range attacks. Long Range Materia will be useful.
| Statistics | ||||||||
| HP | MP | Attack | Mag Attack |
Def | Mag Def |
Dex | ||
| 100,000 | 400 | 100 | 170 | 120 | 100 | 200* | ||
| Elemental Affinity | ||||||||
| Fire | Ice | Lightning | ||||||
| 100% | 100% | 100% | ||||||
| Earth | Poison | Gravity | ||||||
| 0%* | 100% | 0% | ||||||
| Water | Wind | Holy | ||||||
| 0%* | 100% | 100% | ||||||
| Rewards | ||||||||
| Items | Gil | EXP | AP | |||||
| Ultima Weapon | 25,000 | 35,000 | 3500 | |||||
| Other Details | ||||||||
| Morph | Attacks | Steal | ||||||
| None | Ground: Claw Attack, Quake2, Shadow Flare & Ultima Beam Flying: Shadow Flare, Thunderball & Ultima Beam |
Ground: Reflect Ring (Accessory) Flying: Circlet (Accessory) |
||||||
(*) Immune while Flying but does normal damage on ground.
Defeating this Boss is optional but allows you to access the Ancient Forest without needing a Chocobo to climb the mountain and Ruby Weapon will show up at the Gold Saucer.